Horizontal Loop Electromagnetic Research Articles

Integration of geoelectric and electromagnetic methods to delineate subsurface mineralization in bostonite rocks, South El-Atshan area, Central Eastern Desert, Egypt

Bostonite rocks are widely spread in the Central Eastern Desert (CED) of Egypt, they are found in the form of sills or dykes injected in the Precambrian rocks. Postgeosynclinal (Hammamat) sediments were intruded by Postorogenic bostonites. The study of this area, using ground geophysical methods, aimed at tracking the radioactive mineralization under the surface, based on the possible association of this radioactive mineralization with some other mineralizations such as sulphides. The application of the self-potential survey shows that the highest negative anomalies are associated with the South El-Atshan bostonite sill and its contact zones. The depths to the centres of some selected anomalies range from 5.7 m to 24 m, half-widths oscillate from 6 to 32 m, with shallow dips towards the west and east directions. The horizontal-loop electromagnetic (HLEM) survey, which was conducted using four frequencies, revealed the presence of two conductive bodies. These conductive bodies possess narrow widths and depths, ranging from 25 m to 27 m, dipping towards the west direction with shallow angles of 25° and 40°, respectively. In addition, the induced polarization (IP) profile shows high chargeability values reaching more than 7.9 mV/V at depths starting from about 10 m to about 35 m. This zone corresponds to a high-resistance zone that may reflect a subsurface disseminated mineralization. The integration of the results obtained using the three techniques makes it possible to determine the most appropriate zones for the development of exploration in the area of investigation.

Imaging indigenous sacred sites: A tale of clay, silt, and sand

When studying indigenous sites, especially sacred sites such as burials, the needs and wishes of the indigenous people are paramount; the site integrity must be respected, and the site must be left intact and undisturbed. Noninvasive, nondestructive geophysical imaging is well suited to such investigations, but suspicion within indigenous communities because of past transgressions is a barrier to widespread use; skepticism is not uncommon, but we have always managed to convince skeptics. We have developed an overview of results from 10 Māori (the indigenous peoples of New Zealand) sites in the South Island of New Zealand: one historic burial site, six modern burial sites that also had historical use, one historic battle site, one prehistoric site that may have been a burial site, and one prehistoric site used for food storage that was tapu (sacred). Our approach was to let them ask us to delineate areas of importance, for example, old burial sites. There are, naturally, processes and protocols, cultural and technical, that we followed. The sites comprised a range of lithologies and soil types: one site where clay soil overlays limestone, five sites where loess (airborne silt) overlays basalt, one set of inland silty soil sites, one site on peat soils overlying sandy gravels, and two sites in coastal sands. The geophysical responses of the sites cluster into three groups: Horizontal loop electromagnetic (HLEM) and magnetic field methods worked well for the clay soil site, and once the effects of the conductive clay response were removed by filtering, ground-penetrating radar (GPR) worked well. HLEM and magnetic results were good to equivocal on the silty and peaty sites, whereas GPR excelled at delineating anomalous features, particularly burials, which yielded clear characteristic diffraction responses. Finally, results for coastal sand sites were disappointing. Such sites appear to be too dynamic to yield useful results.

Ground Electromagnetic and Electric Studies for Um Salim Gold Mine, Central Eastern Desert, Egypt

Horizontal Loop Electromagnetic (HLEM) and Induced Polarization (IP) methods have been carried out within Um Salim gold mine area. The study area reveals special importance, as it comprises rock units, which show some indication of the presence of various mineralization. The result of HLEM has shown significant and well-defined conductive zones are recorded along with the four used frequencies (110 Hz, 440 Hz, 1760 Hz, and 7040 Hz) at the station 2,782,900 and station 2,782,880 of two (HLEM) profiles (596,050 N & 596,240 N) respectively, that may reflect the sources of the conductive bodies are situated at shallow depths continued to considerable depths. The result of IP exhibits a strong anomalous zone centered at stations 140 and 240, with chargeability values ranging (37 to more than 120 mV/V) on chargeability model section, and corresponding to high resistivity on resistivity model section which may reflect considerable surface and subsurface disseminated mineralization. Through the integration between HLEM and IP a core drilling is recommended at station 140 on IP profile which coincides with station 2,782,850 on HLEM profile (596,240 N) to a depth of concerning 100m take a look at the thickness, depth, and grade of mineralization.

Delineation of radiometric anomalies and conductive zones using gamma-ray spectrometric and electromagnetic methods, east Gabal El-Urf area, Northern Eastern Desert, Egypt

Ground gamma-ray spectrometric (GRS) and horizontal-loop electromagnetic (HLEM) surveys were carried out to determine the radioactive anomalous zones and to follow their lateral and vertical extensions in east Gabal (G.) El-Urf area, Northern Eastern Desert, Egypt. This area and its surroundings are mainly covered by basement rocks of Late Precambrian age, represented by quartz diorites and granite porphyry, which are intruded by pegmatites, quartz veins, and basic dykes. These rocks are traversed by dry wadis (dry valleys), which are filled by Quaternary sediments.The ground gamma-ray spectrometric data revealed that the study area possesses a wide range of radioelement concentrations, oscillating from 37 to 2164 μR/h, 0.9–170 ppm, 0.1–312 ppm, and 0.2%–7.8% for total count (TC), equivalent uranium (eU), equivalent thorium (eTh), and potassium (K), respectively. The eU composite image showed that high radioactive parts are mainly associated with the pegmatite rocks.Ground HLEM survey was performed at four frequencies (7040 Hz, 1760 Hz, 440 Hz, and 110 Hz) in the study area. The interpretation of the HLEM data indicated the presence of various conductive zones, mainly associated with pegmatite bodies. The parameters of source causative bodies, such as location, dip angle, width, and conductivity thickness were estimated for three conductive bodies. The calculated depths to the selected HLEM anomalies appear to be shallow, varying from 10 m to 22 m. All the anomalous bodies have dips ranging between 60° and 80° toward the south and west. The widths of the anomalous bodies range from medium conductors (13 m) to broad conductors (40 m or more). The conductivity thicknesses showed a wide range (1.14–72.7 Siemens), with the highest value associated with the highest eU anomaly.

Parameter estimation by Differential Search Algorithm from horizontal loop electromagnetic (HLEM) data

We present an efficient inversion tool for parameter estimation from horizontal loop electromagnetic (HLEM) data using Differential Search Algorithm (DSA) which is a swarm-intelligence-based metaheuristic proposed recently. The depth, dip, and origin of a thin subsurface conductor causing the anomaly are the parameters estimated by the HLEM method commonly known as Slingram. The applicability of the developed scheme was firstly tested on two synthetically generated anomalies with and without noise content. Two control parameters affecting the convergence characteristic to the solution of the algorithm were tuned for the so-called anomalies including one and two conductive bodies, respectively. Tuned control parameters yielded more successful statistical results compared to widely used parameter couples in DSA applications. Two field anomalies measured over a dipping graphitic shale from Northern Australia were then considered, and the algorithm provided the depth estimations being in good agreement with those of previous studies and drilling information. Furthermore, the efficiency and reliability of the results obtained were investigated via probability density function. Considering the results obtained, we can conclude that DSA characterized by the simple algorithmic structure is an efficient and promising metaheuristic for the other relatively low-dimensional geophysical inverse problems. Finally, the researchers after being familiar with the content of developed scheme displaying an easy to use and flexible characteristic can easily modify and expand it for their scientific optimization problems.

Geophysical imaging of the post-earthquake structural integrity of flood-control stopbanks, Christchurch, New Zealand

On 4 September 2010, a MW 7.1 earthquake struck the Canterbury region of the South Island of New Zealand. Widespread liquefaction caused major damage to many structures, including the flood-control stopbanks along the lower reaches of the Waimakariri and Kaiapoi Rivers. Additional damage occurred during the subsequent MW 6.2–6.3 earthquakes of 22 February and 13 June 2011. Repeated LiDAR surveys indicated that up to 1 m of subsidence occurred in places. Visual inspections identified areas of significant damage, which have been repaired. However, internal damage to the stopbanks cannot be recognized by visual inspection. Thus electromagnetic (EM) and ground penetrating radar (GPR) surveys were undertaken.A pilot study was completed upstream of the confluence of the two rivers, along the northeast segment of the Waimakariri stopbanks and the southwest section of the Kaiapoi stopbanks. A complementary horizontal loop EM (HLEM) survey was carried out in advance of the GPR surveys. The HLEM measurements were done with the instrument oriented both parallel and perpendicular (transverse) to the stopbanks. Anomalous HLEM responses were noted at one location; subsequent GPR surveys indicated a change in the style of stopbank construction and repair, and possibly some internal cracking. HLEM readings were also taken at high and low tide levels along the tidally-influenced lower Kaiapoi River, and significant differences were observed. Finally, during the surveys, a surface crack was observed at one location, and a GPR line across that site suggested that the crack extended to depth.The results were complemented by velocity analyses using subsurface diffractions, and velocity variations were noted along the lengths of the stopbanks. The velocity changes appear to be broadly correlated with changes in the HLEM conductivity, which is not unexpected given the effects of water content and clay content on both the electrical conductivity and the GPR velocity.

HLEM and magnetic surveys: examples from the Orkhun Valley, Mongolia

AbstractArcheologically valuable items and remains that have, somehow, been buried or removed from their original places are the subject of geophysical surveys. Magnetic and horizontal loop electromagnetic (HLEM) methods have been used to locate and limit anomalous regions over large areas. Magnetic data are presented here without applying any modelling process other than simple cleaning of outlines. Median filter and three‐point averaging have been applied to the HLEM data and ratios of measurements parallel and perpendicular to survey lines are presented as maps of the survey area. Results show that different processing techniques produce helpful information, validating the use of different methods over ancient monument areas in Mongolia. Copyright © 2004 John Wiley & Sons, Ltd.

Environmental geophysics at Scott Base: ground penetrating radar and electromagnetic induction as tools for mapping contaminated ground at Antarctic research bases

Mapping the extent of hydrocarbon spills in Antarctica is commonly done by hand sampling of active layer soils and by mechanical coring deeper into the frozen layer. While these methods provide reliable results, the act of obtaining samples can cause almost as much damage to the site as the contamination. A nondestructive, noninvasive method to map extent of hydrocarbon (JP5, JP8) spills on dry permafrost would provide a more environmentally sound contaminated site management tool. This study provides a base knowledge of how noninvasive nondestructive geophysical surveying can be used to map hydrocarbon spills in an Antarctic permafrost setting, and establishes its suitability as a tool for environmental monitoring at Antarctic bases. Geophysical surveys were undertaken at Scott Base, Antarctica, using ground penetrating radar (GPR) and the EM31™ horizontal loop electromagnetic (HLEM) instrument. Surveys were carried out over the same location before and after the summer melt. Areas known to be hydrocarbon-contaminated appeared as resistive anomalies using an EM31. Variations in the geophysical response between surveys were found, with conductivities increasing, on average, by a factor of two for the late summer, but resistive anomalies persisted. GPR was not successful in direct detection of contaminated ground; however, it did provide valuable information on the nature of the subsurface which is useful to predict future movement of contaminants.

Horizontal‐loop electromagnetic signature of a buried dike, Al Quweira area, southwest Jordan

During 1999, horizontal‐loop electromagnetic (HLEM) measurements were made over a buried dike in the Al Quweira area, southwest Jordan, using the APEX MAX MIN III instrument, as part of a mineral exploration project. The objectives of the study were (i) to evaluate the resolution of the HLEM technique in field work in detecting and locating anomalies caused by vein‐like bodies, and (ii) to assess the capability of HLEM surveys for detecting targets in other locations throughout our geophysical survey programme. In‐phase and quadrature anomalies were recorded with 50 m and 100 m coil separations and multiple frequencies across the strike of the buried dike. Data recorded at 43 locations, spaced 10 m apart along the survey line, were interpreted quantitatively. For a 50 m separation, corresponding to shallow depths of investigation, the results do not show any recognizable response from the buried dike. The HLEM data were modelled using a three‐layer structure in order to estimate the thickness of the weathering layer along the survey line. Conversely, data obtained with a 100 m separation, corresponding to moderate depths of investigation, reveal significant anomalies from the buried dike at high frequencies. A phasor or vector diagram was used to calculate the response parameter, depth and dip of the buried dike.

A joint TEM-HLEM geophysical approach to borehole sitting in deeply weathered granitic terrains.

The accurate location of aquiferous fracture zones in granite beneath a > 50 m thick weathered mantle in semi-arid regions is a major hydrogeological problem. It is expected that the zone of intensive fracturing will be more susceptible to weathering and thus be characterized by the thickest development of saprolite, a good electrically conductive target for deep-probing electromagnetic systems. The single-loop transient electromagnetic (TEM) technique is well known to have the capability for detecting concealed steep mineralized targets in mining environments and can be adapted to this hydrogeological problem. We propose that combining the conventional frequency-domain horizontal-loop electromagnetic (HLEM) and single-loop TEM is an effective practical approach to locating concealed aquiferous fracture zones. In the supporting case studies presented here, we deployed multifrequency HLEM profiling (with 50 m transmitter-receiver separation) and TEM soundings with contiguous 10 or 20 m sided loops along the survey lines in a granitic terrain affected by deep (> 50 m) weathering in northeast Brazil. A somewhat layered structure consisting of resistive hardpan/leached zone, conductive saprolite, and resistive basement is identifiable in the typical TEM depth sounding data. We obtained coincident HLEM and TEM anomalies at all the sites, enabling a relatively straightforward selection of potential drilling positions. Simple resistivity-depth transformation of the TEM data was done for each site, yielding an approximate section from which drilling depths were estimated. All of the boreholes located were successful. Although our results appear to indicate that the single-loop TEM method could be used independently for borehole sitting in deeply weathered granitic terrains and that the weathering profile over granite can be mapped using TEM depth soundings of appropriate observational bandwidth, we recommend a joint electromagnetic approach for optimal well sitting.

A Review Of Environmental Applications Of Quasi-Stationary Electromagnetic Techniques

Electromagnetic (EM) techniques are the most commonly used geophysical methods in mineral exploration. However, the use of EM measurements for environmental and engineering applications like the detection of contaminant plumes or the exploration of waste sites is relatively new. The reason for the success of the application of EM methods to environmental problems lies in the variation of conductivity caused by different geometry of pore fluids and clay contents in rocks, and by the presence of organic and inorganic contaminants. Many EM methods/instruments used for mapping near surface geology exist and nowadays they play a central role in environmental geophysics. In general, these methods can be classified in two blocks: EM methods using a plane wave source of excitation and EM methods using a controlled source like a magnetic dipole or a loop source. The Very Low Frequency (VLF, VLF-R) and Radiomagnetotelluric (RMT) methods are chosen as representative methods for plane wave techniques, while horizontal loop EM methods operating in low induction numbers (EM31, EM34) and Transient Electromagnetic methods (TEM) are chosen as representatives of magnetic dipole or loop source techniques. Basic principles, advantages and disadvantages of each technique as well as their connection to specific environmental problems will be discussed. Different successful applications of these methods are reported in the literature. However, this review will focus on three major subjects: waste site exploration, detection of contaminated earth layers, and groundwater exploration. Case histories are presented illustrating the suitability of EM methods for solving such problems.

Detection of subbottom ice‐bonded permafrost on the Canadian Beaufort Shelf by ground electromagnetic measurements

Accurate knowledge of the location of subbottom ice‐bonded permafrost is essential in planning major infrastructure projects such as pipelines in the Arctic. A combination of refraction and shallow reflection seismic surveys and drilling has been considered the most effective means of permafrost detection in offshore environments. Field experiments recently carried out on the Beaufort Shelf by the Geological Survey of Canada indicate that multifrequency horizontal‐loop electromagnetic (HLEM) measurements are a fast and cost‐effective alternative. During test surveys, 60 to 80 soundings were completed in a day. Analysis of HLEM field data shows that a four‐layer model is appropriate for the local geologic conditions. The first layer in the model, sea ice, is highly resistive (5000 Ω ⋅ m). The second layer consists of seawater and seawater‐saturated sediments (estimated resistivity 1 Ω ⋅ m). Because their true resistivities are not significantly different, it was not possible to separate the two highly conductive layers by inversion. The target of the survey, subbottom ice‐bonded permafrost, was the third layer in the section (resistivity 5000 Ω ⋅ m). The bottom layer is formed by unfrozen sediments (average resistivity 2 Ω ⋅ m). Comparison with seismic and drilling results indicates that detection of ice‐bonded permafrost by the HLEM method is reliable; the outlines of its upper boundary given by seismic and HLEM measurements are in good agreement. At the only borehole available, unconstrained inversion of HLEM data provided an accurate estimate of permafrost depth and thickness, but there is not enough drilling information to objectively assess the reliability of the technique.

An exploration strategy for higher-yield boreholes in the West African crystalline basement

Abstract In Burkina Faso investigations have been carried out to test satellite imagery and electromagnetic (EM) methods as a means of improving the siting of water supply boreholes in fissured rocks of the crystalline basement. The results show that these methods may be used to locate boreholes with above average groundwater yields, and to reduce the failure rates. Geometrical analysis of Landsat-MSS lineaments reveals the relevant regional fracture pattern, the present-day principal stress field and the resulting system of oriented hydraulic conductivities. Structural analysis of the lineament pattern and identification of dry-season-active vegetation on Landsat-TM imagery permit promising locations to be pinpointed for groundwater abstraction. EM-measurements on a two-dimensional grid using the ‘very low frequency’ (VLF) and ‘horizontal loop EM’ (HLEM) methods are used to locate potential fractures on a local scale as a basis for rational borehole siting. The methods, applied separately or in combination, offer an economical approach for use in rural and community water supply, small-scale irrigation and water resources planning. The methods appear to be generally transferable to fissured hard rock aquifers in other regions. The best suited settings are believed to be on cratons and platforms in a semi-arid climate.

A robust integral equation solution for electromagnetic scattering by a thin plate in conductive media

An integral equation solution for electromagnetic (EM) scattering by a thin plate robustly models scattering in either perfectly resistive, very resistive, or conducting host media. Because the solution is not restricted to modeling certain ranges of host conductivity, it can be used to model scattering over the large ranges in conductivity encountered in geophysics. The solution is developed around a pair of coupled integral equations for the scattering distributions on the plate. In one equation, the scattering distribution is the scalar potential set up by the scattered charge distribution. In the other, it is the component of the scattered magnetic field perpendicular to the plate. The equations are solved numerically using the Galerkin method with simple polynomial basis functions. To find the fields scattered by the conductor, the scattered current density is first calculated from the scalar potential and the magnetic field. The scattered fields can then be found by integrating over the scattered current density. To test the solution, we model horizontal loop EM responses with our solution and compare the results with those from two established integral equation solutions. One of these solutions models pure induction and is used to test our solution when the host is perfectly resistive. Agreement with this solution is very good. Comparisons with the other solution, an electric field integral equation, tests our solution when the host medium is conductive. Agreement with the latter solution is good where induction is not too strong: i.e., where the electric‐field solution is known to work well. Our solution therefore can accurately model EM scattering by a plate in a host medium with any conductivity.

A novel ground electromagnetic system

A novel, moving‐source, ground electromagnetic system has been developed. The system, named the “Genie,” has been in routine field use since 1982. The system is based on measurement of the ratio of magnetic field amplitude at two widely separated frequencies. The design is presented and compared with traditional horizontal loop electromagnetic (HLEM) systems. Field survey results show the system is effective in detecting conductors at shallow to intermediate depths in resistive environments. The system is shown to be roughly equivalent in many aspects to conventional HLEM systems with important advantages in areas of moderate to rough terrain.

An investigation of finite‐element modeling for electrical and electromagnetic data in three dimensions

The finite‐element method can be used to solve the differential equations which describe electrical and electromagnetic (EM) field behavior. The equations are, respectively, Poisson's equation and the vector, damped wave equation. The finite‐element equations are derived, in both cases, using the minimum theorem. While both tetrahedral and hexahedral elements may be used for the modeling of the resistivity problem, only hexahedral elements give satisfactory results for the EM problem. A disadvantage of the relatively simple mesh design used in the approach described here is the presence of long thin elements. Such elements have very poor interpolating properties, and they adversely affect the rate of convergence of the overrelaxation technique used in solving the resulting system of linear equations. For the modeling of resistivity data over an earth with one plane of symmetry, the system of equations typically has about 9000 unknowns. About 50,000 unknowns are needed to give a satisfactory solution to an EM problem where the earth has one plane of symmetry. The advantage of solving these problems with a technique such as the finite‐element method is that earths with an almost arbitrary distribution of conductivity can be modeled. On the other hand, an integral‐equation method can be far more cost effective for small inhomogeneities. The results from the resistivity algorithm show the adverse effect of an irregular, conducting, and polarizable overburden on dipole‐dipole, induced polarization surveys. Modeling of a horizontal loop EM survey illustrates the importance of assessing the host rock conductivity before attempting to interpret inhomogeneity responses.

THREE‐DIMENSIONAL INDUCED POLARIZATION AND ELECTROMAGNETIC MODELING

The induced polarization (IP) and electromagnetic (EM) responses of a three‐dimensional body in the earth can be calculated using an integral equation solution. The problem is formulated by replacing the body by a volume of polarization or scattering current. The integral equation is reduced to a matrix equation, which is solved numerically for the electric field in the body. Then the electric and magnetic fields outside the inhomogeneity can be found by integrating the appropriate dyadic Green’s functions over the scattering current. Because half‐space Green’s functions are used, it is only necessary to solve for scattering currents in the body—not throughout the earth. Numerical results for a number of practical cases show, for example, that for moderate conductivity contrasts the dipole‐dipole IP response of a body five units in strike length approximates that of a two‐dimensional body. Moving an IP line off the center of a body produces an effect similar to that of increasing the depth. IP response varies significantly with conductivity contrast; the peak response occurs at higher contrasts for two‐dimensional bodies than for bodies of limited length. Very conductive bodies can produce negative IP response due to EM induction. An electrically polarizable body produces a small magnetic field, so that it is possible to measure IP with a sensitive magnetometer. Calculations show that horizontal loop EM response is enhanced when the background resistivity in the earth is reduced, thus confirming scale model results.