Application of Very Low Frequency (VLF) Electromagnetic method to mineral exploration

Project Abstract

<p><b></b></p><p> The Very Low Frequency Electromagnetic (VLF-EM) method, which enables surveying without contact with the ground, is suitable for ground surveys in a wide area and has been used in mapping geology and in mineral exploration for decades. The technique makes use of signal radiation from military navigation radio transmitters operating in the frequency range of 15-30 kHz. When the electromagnetic wave impinges on the surface it is both reflected back into the air and refracted into the earth. By measuring the shifted reflected magnetic field relative to the primary field, subsurface structures can be constrained. Freely and readily available primary signals anywhere around the earth make the VLF method very convenient and efficient for field data collection. Further, VLF data processing using digital linear filtering is quite accurate and very efficient in depicting the qualitative information about subsurface conductors, even though quantitative interpretation of VLF data is as complex as other EM data interpretation. In this study, various aspect of VLF-EM method such as basic theory, worldwide VLF transmitters, and quantities measured and a case study review of the integrated geophysical exploration for sulphide minerals In the Wadi Sa'al area, south Sinai in Egypt (only the VLF-EM method used in this study were review). <br></p>

Project Overview

<p> I. INTRODUCTION&nbsp;</p><p>Exploration helps scientists to understand the earth and its interior. Exploration Geophysical method, aims at detecting or inferring the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, ground water reservoirs and other geological structures using surface methods to measure the physical properties of the earth along with the anomalies in these properties (Alisa,1990). The VLF-EM method of geophysical survey offers a relatively fast approach to delineate the fractures and this fractured zone has high conductivity which could be zones of mineralization or water aquifer (Benson, et al 1997). Geology of the study Area The study area lies between latitude 070 05’N and 070 10N’ and longitude 060 11’E and 060 22’E. Its elevation ranges from 107m to 233m. The area is underlain by Cretaceous and Tertiary sediments of the Nkporo(Campanian) to Nsukka (Maastrichtian to Danian) Formations. The Campanian Nkporo Formation is essentially marine sediments of the third transgressive cycle. The basement rocks outcrops in Igarra area. The location and geological map of the study area is shown in fig. 1. <br></p><p>Fig 1: Location and geologic map of the study area. Materials and Methods The VLF-EM method is an inductive exploration technique that is primarily used to map shallow subsurface structural features in which the primary electromagnetic (EM) waves induce current flow (Karous, and Hjelt,1983; Sinha, 1990; Karkkonen and Sharma, 1997). In principle, it utilizes transmitters operating between 15khz to 25khz as the primary EM wave source. Ground and airborne very low frequency electromagnetic (VLF-EM) surveys have been used successfully to delineate electrical conductors and map geological contacts (Olurunifemi et al 2004; Sharma and Baranwal, 2005). VLF survey or measurement are made utilizing some special military communication transmitter which is located several kilometers away at the high powered military communication transmission stations. The signals generated can travel long distance and able to penetrate the subsurface to induce eddy current in buried conductors. The technique measures the components of very low frequency EM field which are related to the geo-electric structure of the subsurface (Saydam,1981).The electromagnetic method measures the bulk conductivity of subsurface material beneath and between the instrument transmitter and receiver coils. The readings are commonly expressed in the conductivity units of milli-ohms/meter/m-ohms/m or millisiemens per meter (ms/m) EM surveys are used for locating subsurface zones of highly fractured bedrock, buried steel drums and tanks, plumes of groundwater contamination, and clay rich horizons (McNeill,1990; Onu, and Ibe,1998) . VLF magnetic field measurement makes use of E-Polarization in which a transmitter is selected in the direction of strike and measuring profiles are taken perpendicular to the strike direction. Generally, the vertical and horizontal components of magnetic fields are measured and real and anomalies are computed using the expression given by Sinha (1990). Tan 2∝=± 𝐻2 𝐻𝑥 𝐶𝑂𝑆 âˆ†Ø 1− 𝐻2 𝐻𝑋 2 Where ∝ is the dip angles, e is the ellipticity, H2 &amp; Hx are the amplitudes, the phase difference, âˆ†Ø = Ø2-Øx , in which, Ø2 is the phase difference of H2, ØX is the phase of Hx, But, ØL =H2 ei∆ Ø sin ∝ + Hx cos ∝ The tangent of the tilt angle is a good approximation of the radio of the real component of the vertical secondary magnetic field to the horizontal primary magnetic field. The ellipticity is a good approximation of the ratio of the quadrature components of the vertical secondary magnetic field of the horizontal primary field (Paterson and Ronka, 1997). The quantities are called the real (Tan=x100%) and imaginary (=ex100%) anomalies, respectively and they are normally expressed as percentage (Telford, et al, 1976, Lowerie;1997, Saydam,1981). VLF data were collected using an Abem-Wadi instrument. The instrument was fitted to the operator at the base station facing the direction to be surveyed and the system was switched on. It scanned through to pick the most suitable transmitter located roughly perpendicular to the profile. The orientation, azimuth, signal strength and frequency measurement were recorded before the proper measurement were taken. A total of three profiles were established and readings were taken at intervals of 20 meters along each profile. The data were acquired under a favourable weather condition for the three profiles. The first traverse of 1000 meters (1km) was established along Auchi / Fugar road moving from east to west. The second transverse, 1900 meters(1.9km) along Auchi / Okene road moving from southwest to Northeast and the third of 1280meters(1.28km) traverse was located along Auchi / Igarra road moving from west to East. The results were recorded under columns of raw real, filtered real measurement with their corresponding coordinates and elevations. II. RESULTS AND DISCUSSIONS The VLF-EM data were presented as EM profiles, showing plots of raw real and filtered real values against distance (stations). The data were plotted using relevant computer programs. The real component data which are usually more diagnostic of linear features were processed for qualitative interpretation. This enabled the identification of profiles where positive amplitude of filtered real crossover the inflection points of the raw real as points of anomaly. The filtered real transform every genuine crossover or inflection points of the real Application of Very Low Frequency… <a target="_blank" rel="nofollow" href="http://www.theijes.com">www.theijes.com</a> The IJES Page 9 anomaly to positive peaks while reverse crossover become negative peaks. The real and imaginary components are designated as conductance measured in Siemens and the distance measured in meter. In most earth materials, the conduction of electric current takes place virtually entirely in the water occupying the pore space or joint spaces, since most soil and rock forming minerals are essentially non-conductive. Clays and a few other minerals, notably magnetite, carbon and other metallic sulfides may be found in sufficient concentration to contribute measurably to the conductivity of soil or rocks. With reference to the geology of the study area, high conductivity may be as a result of metallic ores/materials, mineralized zones or water aquifer while low conductivity in the area may be as the result of hard rock like intrusive. Also in fractured zones for basement areas, conductivity is always higher than other areas. Since the area is purely sedimentary, shale and sandstone are a bit conductive. Low conductivity values suggest rock unit of high resistivity values like basements rocks and intrusives. The signals from the VLF equipment picked the underlain rocks of the intrusive. Therefore, correlating the geology and the conductivity variations from the signal responses from the VLF results to the electrical properties of rocks which shows that metals have higher conductivity than shale while very high resistivity indicates hard rocks (intrusive rocks). From the image produced using the sufer software, the violet to bluish in the colour code of the VLF values indicates intrusive, greenish to yellowish colour represents the host rocks of sands/sandstone while the reddish to pinkish represents shales rocks. PROFILE A. The trend of the profile is in the west to east direction (N120oE), between latitude 7.145oN to 7.150 N and longitude 6.194oE to 6.2040E(Fig1). It suggests a massive intrusive along the profile with two areas of possible fracture/conductive zones that gave high conductivity value. The first area is between 100m and 140m with conductivity range of (7.6 to 16.8) Siemens from the starting point. The second fractured /conductive zone of mineralization or water aquifer occurred from 420m to 460m with conductivity range of 11 Siemens to 22.5 Siemens. Along the profile, some areas between these high conductivity zones have low conductivity values in the range of 3 Siemens to 6 Siemens. This implies that along the profile from 180m to 300m has low conductivity values ranging from 4.8 Siemens to 5.6 Siemens. The section from 500m to 1260m has low conductivity of range between 3.5 Siemens to 4.6 Siemens but at 760m, high conductivity value of 9.4 Siemens which is likely to be part of the fractured zone. From fig1B displayed, the host rock which is predominately made of sands/sandstone occupies most of the area. The intrusive rocks are located at 120m (lat 7.150230N and long 6.195050 E), 240m (lat 7.149570N and long 6.195870 E) and 920m (lat 7.146250N and long 6.200720 E). PROFILE B. From the image produced, the profile lies between latitude 7.0990N to 7.1020N and longitude 6.3570E to 6.3640E (Fig 2). The trend of the profile is in the east to west direction. Along the profile between 480m and 500m, a high conductivity value of 13.2 siemens was obtained. Other areas that have low conductivity in the range of 2.0 Siemens to 5.2 Siemens. Low conductivity values suggest rock units of high resistivity values like basement rocks and intrusives. This implies that along the profile, the first 400m has low conductivity values ranging from 0.9 Siemens to 4.3 Siemens. The second section occurred from 520m to 1000m, with low conductivity values in the range of 0.9 Siemens to 5.2 Siemens. The profile is occupied by the host rock which is made up of predominately sands/sandstones. In fig. 3, the violet-bluish area which indicates a conductive zone, occurred at about 380m (lat 7.101520N and long 6.361470E) and from the remark, it indicates the presence of a river. PROFILE C. The trend of the profile is in the Northeast –Southwest direction N500 E direction (Fig. 4). Profile C suggests a massive intrusive along the profile with two areas of possible fractured zones that gave high conductivity of about 50 Siemens at 300m from the starting point. The second fractured zone of mineralization or water aquifer occurred from 900m to 1100m with conductivity range of 35 Siemens to 50 Siemens. Other areas between these zones have low conductivity values in the range of 4.0 Siemens to 7.0 Siemens. This implies that along the profile, the first 200m has low conductivity values ranging from 1.9 siemens to 8 Siemens. The second section from 380m to 880m also have low conductivity values in the range of 0.3 Siemens to 8.7 Siemens and the third section from 1100m to about 2000m have range of values from 0.9 Siemens to 9.4 Siemens except at 1140m with value of 15.4 Siemens which is likely to be part of the fractured zone. From fig. 5, violet to bluish colour indicates the intrusive, greenish to yellowish colour represent host rocks of sands/sandstone while the reddish to pinkish colour denotes shale rocks. From the result obtained, the host rocks are predominately sands and shales. The intrusive rocks are located in the mid of the profile about 1000m (lat 7.1560N and long 6.3280E) and in the lower range between lat 7.1480N and long 6.32950E<br></p>