Using electromagnetic methods to map and delineate high-grade harzburgite pods within the Ni-Cu mineralised Jacomynspan ultramafic sill, Northen Cape, South Africa

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2016.The Jacomynspan Ni-Cu sulphide mineralisation is hosted within a 100m thick steeply dipping tabular, differentiated, sill of mafic to ultramafic composition intruded into country gneissic rocks of the Namaqualand Metamorphic complex. This sill is predominantly composed of tremolite schist (metamorphosed pyroxenite) containing lenticular bodies of harzburgite. The harzburgite generally hosts net-textured mineralisation with up to 50% by volume of the rock. Massive sulphide veins and stringers are occasionally present within the harzburgite. The sulphide minerals are a typical magmatic assemblage of pyrrhotite, chalcopyrite and pentlandite. The sill covers an approximate strike length of about 5km but only a small portion covering 1km x 1km was selected for this study. Physical property studies carried out on the drill core (magnetic susceptibility and conductivity) indicate that the country gneissic rocks are not conductive and neither are they magnetically susceptible. However, the mineralized sill has elevated values of both magnetic susceptibility and relative conductivity compared to its host making it a suitable target for both magnetic and electromagnetic inversion. Drilling done so far on the study area has shown that the well-mineralised harzburgite (hosted within the poorly mineralised ultramafic sill) is not a continuous body but occurs in ‘pockets’. There is therefore need to use the available geophysical and geological datasets to derive a model of these well mineralised pods. This study is therefore intended to assess the feasibility of using electromagnetic (EM) methods together with other geophysical methods and geology in obtaining a model of the harzburgite pods hosted within the less conductive poorly mineralised ultramafic sill in order to guide further drilling. Geosoft’s VOXI Earth Modelling software was used to model the high resolution airborne magnetic data for this study. Cooper’s Mag2dc (www.wits.ac.za) and Stettler’s Magmodintrp software (personal communication, 2015) was also used during modelling of the magnetic data to compliment the modelling from VOXI. The mineralised ultramafic sill was clearly mapped in both the 3D model representation from Mag2dc modelling and VOXI’s 3D unconstrained smooth model inversion for the study area. Based on the physical properties studies carried out on the study area, EM data (both ground and downhole EM) were modelled using Maxwell software. The poorly mineralised tremolite schist was clearly modelled. In order to better constrain the targets, an assumption was made that at late decay times the currents would be focused in the centre of the large EM plate probably giving an indication of the most conductive part of the intrusion. Smaller ‘Resultant EM plates’ of dimensions, 300mx300m that coincide with the centre of the large EM plates (with a conductance above 100S) were constructed in iv Maxwell software and integrated with the DXF file of the Micromine geology model of the well mineralised harzburgite clearly mapping the well-mineralised harzburgite and showing its possible extensions. 2D inversion modelling was conducted on all audio-frequency magnetotelluric (AMT) data for this study area. The modelling results clearly mapped the mineralised intrusion

Forward modelling 3-D geophysical electromagnetic field data with meshfree methods

Simulating geophysical electromagnetic (EM) data over real-life conductivity models requires numerical algorithms that can incorporate realistically complex geometry and topography. The most successful way to incorporate them is to use unstructured meshes in the discretization of an Earth model. Current mesh-based numerical methods that are capable of using such meshes have inherent drawbacks caused by generating 3-D unstructured meshes conforming to irregular geometries. Such a mesh generation process may become computationally expensive and unstable, and particularly so for EM inversion computations in which the forward modelling may be required many times. In this thesis I investigate the feasibility and applicability of radial basis function-based finite difference (RBF-FD), a meshfree method, in forward modelling 3-D EM data. In the meshfree method, the physical model is represented using only a set of unconnected points, effectively overcoming the issues related to the mesh generation. To improve numerical efficiency, unstructured point sets are used in the computation for the first time for EM problems. The computation is further accelerated by introducing a new type of radial basis function in the RBFFD method. The convergence and accuracy of the proposed RBF-FD method are demonstrated first via forward modelling gravity and gravity gradient data. The computational efficiency of the meshfree method is compared with that of using a more traditional finite element method. The meshfree method is then applied to forward model magnetotelluric data of which the effectiveness is demonstrated using three benchmark conductivity models from the literature. Faithful reproduction of the physics in the EM fields, e.g. discontinuous electric fields across the conductivity contrasts, is achieved by proposing a hybrid meshfree scheme which is a modification to standard meshfree algorithms. The hybrid method is also applied to simulate controlled-source EM data in the frame of both total-field and primary-secondary field approaches, in which the problems in dealing with singular source functions that cause singularities in the EM fields are addressed. For these two approaches, the accuracies of the proposed hybrid meshfree method in forward modelling the controlled-source EM data are demonstrated by using idealized 1-D layered models and a 3-D marine canonical disk model. The successful applications of the proposed meshfree method in modelling the above EM data suggest that the meshfree technique has the potential of becoming an important numerical method for simulating EM responses over complicated conductivity models

Capacitive sensors for measuring complex permittivity of planar and cylindrical structures

With the increasing use of low-conductivity structural and functional materials, there has been a greater need for the efficient and reliable nondestructive evaluation (NDE) of these materials. One approach to evaluate low-conductivity structural and functional materials is to characterize the material dielectric property. In this thesis, capacitive sensors are developed for measuring complex permittivity of planar and cylindrical materials. For each sensor configuration, models are developed to allow for inverse determination of material permittivity from measured capacitance, therefore realizing quantitative characterization of material dielectric properties. In the first half of the thesis, coplanar concentric capacitive sensors are developed to meet the need of detecting water or excessive inhomogeneities caused by repairs in aircraft radome structures. Another important motivation is the absolute dielectric property characterization of laminar structures. Three coplanar sensor configurations are designed: the simple two-electrode concentric configuration, the interdigital spiral and the interdigital concentric configurations. Corresponding numerical models are developed to predict the sensor capacitance for given test-piece structures. The validity of the models is verified by comparing numerical predictions and measurement results. The advantage and disadvantage of each sensor configuration is discussed. For the two-electrode concentric configuration, a prototype handheld probe is also fabricated, and has detected successfully 1 cc of low contrast liquid in a simulated radome structure. Curved patch capacitive sensors, presented in the second half of the thesis, are developed with the motivation of accurate and convenient permittivity measurement of cylindrical structures. It is demonstrated that the permittivity of homogeneous dielectric rods is inferred easily from measured sensor capacitance, based on analytical and numerical models developed here. Another practical application of the curved patch capacitive sensors is the quantitative evaluation of aircraft wiring insulation condition. In this work, wires are modeled as cylindrical dielectrics with a conductive core. A numerical relationship between the complex permittivity of the insulation and the sensor capacitance and dissipation factor is established. A prototype probe, developed based on this model, has distinguished successfully degraded wires from the control ones. The feasibility of utilizing the presented capacitive approach for quantitative evaluation of aircraft wiring insulation condition is demonstrated. Although the development of the capacitive sensors in this thesis is motivated by aerospace engineering related applications, results presented in this work have the potential to be applied to other engineering fields. Potential sensor applications and recommended future research are suggested at the end of the thesis

The Capacitive Resistivity Technique for Electrical Imaging of the Shallow Subsurface

Capacitive resistivity (CR) is a novel geophysical technique for the non-intrusive characterisation of the shallow subsurface by electrical imaging. CR is capable of extending the scope of the conventional DC resistivity technique to the urban built environment and other settings where galvanic contact cannot be achieved or where high contact impedances result in poor data quality. Fundamentally, the CR technique is based upon the concept of capacitive coupling between sensors and the ground and a generalisation of the DC four-point array for measuring the resistivity of the subsurface at frequencies in the VLF range. This thesis provides a unified description of CR, including its physical principles, their theoretical formulation and practical implementation in geophysical instruments. In general, the transfer impedance across a capacitive array is a complex function of frequency and geometry. It is shown that a low induction number mode of operation exists where resistivity is proportional to the in-phase component of the transfer impedance. The quadrature component is generally sensitive to a combination of parameters including sensor elevation, dipole offset and ground resistivity. Under the low induction number regime, the electric field is quasi-stationary so that theoretical equivalence with the DC case is achieved and conventional DC interpretation schemes are applicable to CR data. A comprehensive parameter study undertaken in this thesis investigates the applicability of the technique under the specific conditions typically encountered in environmental and engineering site investigation surveys. In those circumstances, practical CR measurements are shown to be limited to an optimal frequency window between 1 kHz and 25 kHz. The condition of low induction numbers imposes further restrictions on the maximum dimensions of the sensor array and the minimum resistivity of the ground. However, a key finding of the parameter study is that even under the quasi-static regime, practical conditions may be such that substantial phase rotations may occur which are exclusively due to the capacitive nature of the technique. Modelling of sensor capacitances is used to demonstrate that the concept of point poles postulated in the quasi-static formulation of CR has a practical realisation in the form of plate-wire sensors. Subsequently, the fundamental concepts of CR are validated experimentally in a series of elementary surveys where the fully complex transfer impedance (amplitude and phase) is measured with a newly developed prototype CR instrument. It is shown that for assessments of shallow subsurface conditions with typical survey parameters and standard geometries, the observed responses are typically in-phase. However, it is also demonstrated that practical circumstances exist under which significant phase rotations can be observed. In such cases, an estimation of apparent resistivity using the in-phase component only is more appropriate than the magnitude-based calculation performed by existing commercial instruments. The nature of the CR technique facilitates the use of towed arrays that allow the dynamic collection of multi-offset apparent resistivity data without the disadvantages of galvanic coupling. This thesis examines the operational characteristics of towed CR arrays and compares data acquired with a range of instruments in a variety of different environments. It is shown that towed-array CR enables the collection of highly repeatable resistivity data at sampling intervals of the order of centimetres. Towing-induced noise is found to be much less problematic than previously found with DC towed-array techniques. It is also demonstrated that high-quality data can be obtained by towed-array CR on artificial surfaces such as tarmac or concrete. Consequently, the technique appears to be particularly suited for assessing the condition of engineered structures such as roads and pavements. Finally, it is demonstrated how multi-offset towed-array CR can be employed for electrical tomographic imaging of the shallow subsurface. Conventional DC resistivity interpretation schemes based on quasi-2D, 2D and 3D inversion algorithms are shown to be applicable to such datasets, provided that some elementary rules are observed with regard to the design of towed-array surveys. Real-time interpretation during data acquisition is shown to be feasible with a continuous vertical electrical sounding (CVES) technique based on a Zohdy-type inversion. Examples of 2D and 3D surveys over shallow targets show the superior quality and resolution of CR datasets compared with conventional DC resistivity

Forward modeling of magnetotellurics using Comsol Multiphysics

NSERCPeer ReviewedMagnetotellurics is an electromagnetic geophysical method that has been widely used to study structures in Earth's subsurface. Numerical modeling of magnetotellurics is important for survey design, inversion, geological interpretation and many other aspects of geophysical studies. For example, modeling a subsurface conductive body in terms of its conductivity, geometry and dipping angle would yield substantial information on the phase response and sensitivity in an MT survey. While there are many different modeling techniques, the finite element method is most commonly used. In this effort, we present magnetotelluric models of layered Earth, uplift structures, auroral electrojets, and geomagnetically induced currents in power-line skywires using the commercial finite-element package Comsol Multiphysics. The AC/DC module in Comsol can be used to solve Maxwell's equations in the quasi-static limit for modeling the magnetotelluric response. One of the advantages of Comsol modeling is its Graphical User Interface (GUI), which allows users to solve complex single or multi-physics problems in a meshed domain. The use of Comsol also reduces the need for sophisticated computer coding when solving partial differential equations such as Maxwell's equations. In the effort presented here, we first discuss model validation for layered Earth geometries. We then present two examples of magnetotellurics modeling in impact crater and geomagnetically induced current studies. Numerical results were compared with analytical solutions or benchmark results whenever possible

Multi-dimensional Resistivity Models of the Shallow Coal Seams at the Opencast Mine 'Garzweiler I' (Northwest of Cologne) inferred from Radiomagnetotelluric, Transient Electromagnetic and Laboratory Data

The entire Cenozoic unconsolidated fill of the Lower Rhine Embayment in Germany hosts the largest single lignite, or brown coal, deposit in Europe which covers an area of some 2,500 km2 to the northwest of Cologne. Rhineland brown coal is mined in large-scale opencast mining and accounts for around one-quarter of the public electricity supply in Germany. The present study was devoted to carrying out radiomagnetotelluric (RMT) and transient electromagnetic (TEM) investigations over the shallow coal seams at the opencast mine 'Garzweiler I.' The main objectives of the survey were to highlight the applicability and efficiency of RMT and TEM methods in an area like brown coal exploration, and to image the vertical electrical resistivity structure of these coal seams. Therefore, the vertical and lateral resolution capabilities of such methods were as necessary as the ability to cover large areas. Consequently, a total of 86 azimuthal RMT and 33 in-loop TEM soundings were carried out along six separate profiles over two opencast benches at the 'Garzweiler I' mine. The local stratigraphy at the survey areas comprises a layer-cake sequence, from top to bottom, of Garzweiler, Frimmersdorf and Morken coal seams embedded in a sand background, consisting of Surface, Neurath, Frimmersdorf and Morken Sands. A considerable amount of clay and silt intervenes the whole succession. The data were interpreted extensively and consistently in terms of one-dimensional (1D) RMT and TEM resistivity models, without using any complex multi-dimensional interpretation. However, the presence of thin, surficial clay masses (or lenses) broke down such interpretation scheme. In this case, to greatly improve the resistivity resolution for these surficial masses and the underlying coal seams, two-dimensional (2D) RMT and three-dimensional (3D) TEM interpretations have been carried out. They could be used effectively to study the local EM distortion on the measured data, where these surficial masses were found, as well as to cross-check the nearby-topography effect. Because the RMT data are usually skin-depth limited, they only provided a resolution depth between 25 and 30 m for the shallow resistivity structures. Whereas, the TEM data still have sufficiently early- to late-time information, and therefore resulted in a better resolution depth of about 100 m for the shallow to sufficiently-deep resistivity structures. The final 1D/2D RMT and 1D/3D TEM resistivity models displayed a satisfied correlation with both thicknesses derived from the stratigraphic-control boreholes and resistivities measured from direct-current (DC) and spectral induced polarization (SIP) laboratory techniques on 16 rock samples. As demonstrated, the integrated use of azimuthal RMT and in-loop TEM soundings was highly successful and effective at mapping the major stratigraphic units at the survey areas, i.e. the shallowest conductive Garzweiler and Frimmersdorf Coals within their fairly resistive sand background. They could not distinguish between Neurath Sand and the underlying sand/silt or between Frimmersdorf Coal and the underlying organic clay. The deepest Morken Coal was beyond the depth-of-investigation of the present measurements. Finally, the resistivity models revealed that both coal seams gently dip in the southwesterly direction. This should be in fairly good agreement with the regional structural makeup of the Rhineland brown coal. However, they showed that Garzweiler Coal is gradually thinned northeastwards, while Frimmersdorf Coal still has almost a regular thickness

Theoretical Developments in Electromagnetic Induction Geophysics with Selected Applications in the Near Surface

Near-surface applied electromagnetic geophysics is experiencing an explosive period of growth with many innovative techniques and applications presently emergent and others certain to be forthcoming. An attempt is made here to bring together and describe some of the most notable advances. This is a difficult task since papers describing electromagnetic induction methods are widely dispersed throughout the scientific literature. The traditional topics discussed herein include modeling, inversion, heterogeneity, anisotropy, target recognition, logging, and airborne electromagnetics (EM). Several new or emerging techniques are introduced including landmine detection, biogeophysics, interferometry, shallow-water electromagnetics, radiomagnetotellurics, and airborne unexploded ordnance (UXO) discrimination. Representative case histories that illustrate the range of exciting new geoscience that has been enabled by the developing techniques are presented from important application areas such as hydrogeology, contamination, UXO and landmines, soils and agriculture, archeology, and hazards and climat