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6th International AEM Conference & Exhibition
- Conference date: 10 Oct 2013 - 11 Oct 2013
- Location: Kruger National Park, South Africa
- Published: 10 October 2013
41 - 60 of 69 results
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VTEM and ZTEM helicopter EM case-study over the Nuqrah Cu-Pb-Zn-Au sedex massive sulphide deposit in Western Arabian Shield, KSA
Authors J.M. Legault, A. Prikhodko, C. Izarra, S. Zhao and E.M. SaadawiHelicopter VTEM active source, ZTEM AFMAG passive source EM and aeromagnetic survey results are compared over the Nuqrah Sedex massive sulphide deposit in Western Arabian Shield of KSA. Field data and 1D-2D- inversions are used to show that all surveys map major controlling structures that host the Nuqrah deposits. VTEM directly detects more massive sulphide mineralized vent portions of Sedex orebodies; whereas ZTEM likely defining larger, less conductive and weakly mineralized distal portions of Sedex system. ZTEM also maps possible conductive down-dip extension of Nuqrah South Sedex below 750m depths.
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Time Domain Helicopter EM System Equator: Resolution, Sensitivity, Universality
Authors E. Moilanen, E. Karshakov and A. VolkovitskyTime domain helicopter borne electromagnetic system EQUATOR was developed in 2010. For past years it worked successfully on survey in various conditions. It proved its effectiveness for searching low contrast respectively small targets such as disseminated ore, kimberlites. EQUATOR’s main features are wide survey speed range and wide frequency band of receiver. First is due to construction properties. Speed variations do not affect survey results due to high precision transmitter-receiver positioning. Second advantage extends system abilities. Due to full time measurements EQUATOR provides also frequency-domain data processing. To get more informative frequency response several additional frequencies are induced. Added signal doesn't affect time-domain response because it is absent in operating impulse spectrum and can be removed in post processing. Due to small footprint EQUATOR is well suited for small sized targets exploration. Its’ measurements correspond well with ground EM and drilling data.
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Modelling the superparamagnetic response of AEM data
Authors Daniel Sattel and Paul MuttonSeveral lines of airborne EM data flown at different system elevations across a known sulphide and surface cover with elevated superparamagnetic (SPM) properties were analysed with MAXWELL, layered-earth inversions and LEROIAIR. The SPM material was modelled with frequency-dependent magnetic susceptibilities at shallow depth. The SPM responses can be confused with responses of deep conductors and vice versa. Depending on the parameter weighting used, 1D inversions model all late-time responses as deep conductive material or as surficial SPM material. However, the joint 1D inversion of data acquired at different system elevations manages to recover a deep conductor from the sulphide anomaly and elevated SPM values at the location of the SPM response. For the modelled parameters, the VTEM data require a vertical system separation of at least 5-10 m to allow for the discrimination between the SPM and sulphide responses. In the absence of vertically separated AEM profiles, the x-component data, if available, might offer some model discrimination. For laterally extensive surficial SPM material, synthetic x-data computed with a modified version of LEROIAIR show a negligible SPM response. Following the determination of SPM parameters from VTEM survey data, these values were used to predict the SPM response for other AEM systems, including MEGATEM, AEROTEM, HELITEM and RESOLVE. Whereas the transient helicopter EM systems VTEM, AEROTEM and HELITEM data can be strongly affected by SPM effects, fixed-wing MEGATEM data are unaffected, due to the high elevation and large transmitter – receiver separation of the EM system. SPM effects on frequency-domain systems such as RESOLVE data are also small.
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Application of AEM to shallow geothermal potential mapping
Authors Andrea Viezzoli, A. Menghini, A Manzella, D Montanari, S Maggi and A SantilanoThe 'VIGOR' project aims at assessing geothermal resources in 4 regions of Southern Italy. The SkyTEM survey is a part of the planned geophysical activities and has been chosen since it is able to collect resistivity data on wide areas, with great resolution and in short times. To date, the application of AEM methods to geothermal targets has been limited, due to the depth at which the target is usually found. Kirsh and Siemon (2008) suggested its application in low enthalpy study. In this project AEM is tested in an area of known geothermal activities, and where a wealth of ancillary data is also present, in western Sicily. The results are composed in a 3D model of the electrical resistivity of the subsurface, and then interpreted for geothermal potential, following 2 main paths. The first is to produce a model of heat exchange capacity. The second aims at identifying directly conductive anomalies that might be associated to locally shallow hydrothermal fluids. “Termini” area is covered by extensive geological surveys. Since geological conditions of Sicily, even at shallow depth, are very complex, this area provided a good place for defining the resistivity values of the main geological units outcropping in the region. The “Western Sicily” area covers the main thermal manifestations of Western Sicily. Based on near real time preliminary results from the more regional mapping at 1 km line spacing, three infill areas were selected as being the most promising to test the concept. The obtained resistivity volume has then been the base for a detailed lithological and geothermal interpretation. Lithological and geological maps were used to constrain surface condition and to understand the resistivity ranges of the different lithological units. On the base of AEM derived resistivity values, and of laboratory measurements of thermal and electrical conductivity on samples, it was possible to establish the main links between lithology, electrical resistivity and thermal conductivity. The work in progress aims at extending the correlation also at depth, producing a 3D model of thermal exchange capacity for the areas surveyed. This detailed interpretative modeling provides also the basis for detecting resistivity anomalies within carbonate units, which may possibly represent hydrogeological or hydrothermal bodies.
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Modelling the electromagnetic response of the helicopter-borne ZTEM system for vertical thin plate conductors buried below conductive overburden
Authors G. Connors, C. Samson, J.M. Legault and S. ZhaoA desire to reduce the need to perform forward modelling on a case by case basis has resulted in an endeavour to model the ZTEM response and produce type curves. A total of 42 models of the subsurface were analysed using the OCCAM 2D MT forward modeling code. These models investigate two types of thin-plate conductors; conductive overburden and a conductive mineralized dyke. The conductance of the target, and overburden ranged from 100,000 S to 0.01 S, and 10,000 S to 0.01 S respectively. Modelling and graphical representation of the synthetic responses allowed for the production of type curves. One type curve was created for each of the six frequencies (30 Hz, 45 Hz, 90 Hz, 180 Hz, 360 Hz, and 720 Hz) that ZTEM measures. As expected, lower frequencies were generally more effective at resolving the conductive dyke below conductive overburden to depths down to 1000 m. A steady decrease in a ZTEM response as frequency increased was usually observed. The amplitude of the tipper response generally increased with the conductance of the dyke and decreased with the conductance of the overburden. The type curves produced for each frequency ultimately represent a useful exploration tool that can be used to predict the likelihood of successful performance of ZTEM airborne surveys for conductive targets in different geological environments around the world.
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3D geological modelling using geological and EAM data
Authors J. Deparis, B. Vittecoq, T. Jaouen, F. Lacquement and J. PerrinUnderstanding the hydrogeological functioning of volcanic island is essential for an adapted management of water resource and for a better supply to population, particularly in those who have seen their population increase in years. Nevertheless, geological, hydrological or hydrogeological data are often very scarce, and appropriate approach have to be found in order to improve the hydrogeological functioning of these islands. Groundwater resource is closely correlated with the permeability of rocks, and, in basaltic volcanic islands, permeability is usually considered to be negatively correlated with age formations. Younger lavas have higher permeability than older one’s often highly weathered (Custodio et al., 1988). Existing hydrogeological models of basaltic volcanic islands describe large-scale systems at the volcano or island scale, and two end models have been described. The Hawaiian model (Meinzer, 1930) considers a low-lying basal aquifer linked to inland dike-impounded and perched aquifers overlying impervious layers and/or confined by dykes (Tabasaki and Mink, 1983). The Canary Islands model (Custodio, 1975; Custodio et al., 1988) considers a continuous and isotropic basal aquifer, and a decrease of hydraulic conductivity with the age of the volcanic rocks. Nevertheless, those models are consistent with Young Island (< 5 Ma) and seem not fully appropriate for older island such as Mayotte Island. Mayotte Island is a small French overseas territory of the Comoros archipelago in the Indian Ocean. Its population has quickly increased during the last decades and has been multiplied by four in less than 30 years. The island is now very densely populated with more than 500 inhabitants per square kilometer. Water needs are thus substantial. Surface water resource is very strongly solicited, and too dry seasons can lead to critical situations. Nevertheless, its hydrogeological functioning is not well known, and understanding the hydrogeological functioning of the island is thus essential for an adapted management of groundwater resources and to make appropriate drilling campaigns. In 2010, an helborne SkyTEM survey on the whole island has been performed. The transmitter loop is composed of four 284 m² loops that transmit a low moment with a single turn and a high moment with four turns of the loop. The current generated by the low moment is 11 A, giving a moment of about 3100 Am² and a turn off time of 10 microseconds. That generated by the high moment is 108 A, giving a moment of about 123,000 Am² and a turn off time of 38 microseconds. The first sampling gate is centred about 6.5 microseconds for the low moment and the last window of the high moment is 8 ms. A linear 3000 km of SkyTEM TDEM data was acquired in a month. The average flight line spacing was 200 metres, with local spacings of 400 metres or 100 metres. The average speed of the helicopter was 18 m/s (65 km/h) with an average ground clearance of the transmitter loop being 48 metres. The TDEM decay curves were filtered using Workbench software with 1D modelling of the curves based on the Ward and Hohmann (1988) solutions. Data inversion was done with em1Dinv software (Auken et al, 2004) using a 20-layer model with fixed depths and a spatial constraint between the model resistivities of nearby soundings (Viezzoli et al, 2008). This model was preferred to a stratified-type inversion because the development of the weathering profile is not conducive to the presence of a sharp contrast between the bodies. The aim of the research presented in this paper is to show the calibration of 3D resistivity mapping realized on another and older volcanic island, with boreholes data in a test site in the South of Mayotte Island, and how the highlighted paleo-valley structure enhance understanding the hydrogeological functioning of the South of Mayotte Island. To establish the 3D model, we used correlation between borehole, geological map and 3D resistivity grids. Comparison between TDEM data, borehole and pumping tests data have provided new evidence about the hydrogeological scheme of the island, which seems different from the two previously mentioned. This study is focused in the South of the and includes two watersheds. The surface of the Kani-Kéli watershed, the west one, is about 4.9 km², and the surface of the Mronabeja watershed, the est one, is about 4.4 km². Four boreholes have been made in the past Simplified and interpreted geological logs show that two boreholes are drilled in weathered lavas, whereas the two others encountered about 30 m of weathered lavas overlying unweathered and superposed fresh fissured lava flows, with a confined aquifer in the fresh lava. 3D geological model of this area is made with GeoModeller ® software. Two kinds of data are used: polygons extracted from the provisional geological map using ArcMap and TDEM data. One model was produced with TDEM sounding (i.e. without resistivity interpolatation). 3D modeling of different geological formations can delineate structures with a hydrogeological interest. Indeed, in healthy fractured lavas, characterized by variable resistivity of 30 to 100 Ω.m, are considered as potential reservoirs of groundwater in these formations modeled structures thus correspond to aquifer sector Choungui South. The results show that helicopter borne TDEM is particularly adapted for this volcanic island, with low resistivities soils outcropping. Data computation also allows to image geological structure in 3D and boreholes comparison allows attributing geological identification for the main ranges of resistivity data. Productive boreholes, in the axis of a resistive body, and unproductive boreholes outside, allow confirming the implication of paleo-valley as major constraint for groundwater flows and the necessity to enhance existing conceptual model. The South of Mayotte Island is thus proposed as an end member of basaltic models. These results are very hopeful, and we are now looking for similar structure in order to drill new boreholes, to confirm the replicability of this aquifer structure, and to provide water to population method.
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The Power of Frequency Domain: When you should be using it
By G. HodgesThe characteristics of frequency domain EM system give it a broad frequency range and more than 5 orders of range for resistivity. The smaller size allows a rigid geometry and thereby in-phase data. Resistivity from phase angle is very sensitive across a wide range, and relatively independent of altitude, as well as measuring magnetic susceptibility and dielectric. The smaller size also allows a coaxial (X) component and close terrain following for the best possible lateral resolution. HFEM should be used where near-surface vertical and lateral resolution are essential, and where geology is resistive or targets weakly conductive.
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Recent AEM case study examples using a Full Waveform time-domain system for near-surface applications
Authors A. Prikhodko, J.M. Legault, K. Kwan, T. Eadie, K. Fisk, G.A. Oldenborger, V. Sapia, A. Viezzoli, E. Gloaguen, B.D. Smith and M.E. BestEarly time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas but this poses difficulties due to a variety of issues, such as system bandwidth, system calibration and parasitic loop capacitance. In an effort to address this issue, a continued system design strategy, aimed at improving its early-channel VTEM data, has achieved fully calibrated, quantitative measurements closer to the transmitter current turn-off, while maintaining reasonably optimal deep penetration characteristics. The new design implementation is known as “Full Waveform” VTEM. This paper presents some case-study examples of the Full Waveform VTEM helicopter time-domain EM system for near-surface applications.
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Preliminary design parameters for a HTEM system dedicated to overburden mapping
Authors Raymond Caron, Claire Samson, Michel Chouteau and Martin BatesLarge areas of the Precambrian shield in Canada, such as in the Abitibi mining region, are covered with a thick glaciolacustrine overburden. This overburden complicates mineral exploration efforts by masking the bedrock surface and increasing the cost of exploration through expensive drilling programs. It also reduces the resolution of potential-field exploration methods by obscuring changes in bedrock topography that are known to change independently from the topography at surface. With respect to the gravity method, changes in bedrock topography can also create anomalies of the same size and magnitude of, and be mistaken for, mineral deposits (Chen and Macnae 1997). In this paper, we suggest design parameters for a HTEM system specifically dedicated to overburden mapping with the goal of using the recorded data to correct airborne gravity data for lateral variations in overburden thickness. The proposed HTEM system is a single-turn in-loop system where the transmitter and receiver are in the same plane but offset by 7m. The 7m-radius circular transmitter is energized by a square waveform with a 100 A current running at a maximum duty time duration of 0.5 ms. The transmitter moment is 15,400 A/m2. The parameters were tested using geological scenarios based on the geology prevalent throughout the Abitibi mining region. Previous electromagnetic surveys in this area identified 3 classes of glaciolacustrine sediments above the bedrock. These units of clay, sand, and till have an average resistivity of 47.3 ± 6.7 Ω•m, 251 ± 70 Ω•m, and 123 ± 35 Ω•m respectively (Palacky 1992). The bedrock was assigned a resistivity of 10,000 Ω•m. Forward modelling of the EM response of the proposed system of various 2-layer, 3-layer and 4-layer geological scenarios were conducted and then inverted in order to determine the capabilities and limitations of the HTEM system. Noise was added to the response of the forward model following the process outlined in Auken et al. 2008 in order to simulate field conditions. The inversion methodology takes advantage of the highly resistive character of the bedrock and the typical location of clay above till within the sedimentary sequence. For these simple cases, the system was able to resolve the overall thickness of the overburden within 10%. EM responses of geological scenarios composed of 5 to 8 layers with intercalations of clay, sand and till were inverted, with noise, with the same methodology. For these more complex cases, the tests showed that multiple layers are better resolved when using a similar number of layers in the inversion as in the geological model. However most important is that for any case of layering, the overburden thickness is resolved within 10% of its true value and the difference between the inverted depth and the true depth is randomly distributed. This preliminary study indicates that the proposed HTEM system is likely to be able to resolve the overall thickness of a glaciolacustrine overburden overlying resistive Precambrian bedrock.
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An efficient hybrid inversion scheme combining approximate and full forward solutions for AEM
Authors C. Kirkegaard, C. Schamper, A.V. Christiansen, G. Vignoli and E. AukenAirborne time domain electromagnetic instruments provide a widely used surveying tool for a range of applications with very varying requirements towards accuracy. The method allows for collecting extremely large datasets in relatively little time, but to be able to extract quantitative information the data typically has to undergo some kind of modelling. In mineral exploration it is often possible to identify targets directly from the data, whereas high-accuracy modelling is typically crucial in the case of eg. hydro-geophysical surveys. In order to support the requirements of these very different fields a whole range of modelling schemes exists, ranging from very fast approximate techniques to more accurate and much more time consuming full system models. We discuss how an approximate- and a full system forward model can be combined into a versatile hybrid inversion scheme that makes no compromise in the numerical formulation of the physical system. We use this hybrid scheme to examine different degrees of approximation and demonstrate the impact on accuracy and performance.
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Evolution of TEMPEST – Bird positioning
Authors S. Mulè and R LockwoodTEMPEST, a fixed wing time domain electromagnetic (TDEM) system, developed in 2000, has been applied to a range of exploration and mapping applications around the world. The systems’ versatility can be credited to its broad operational bandwidth, multifaceted software approach and distinctive calibration technique which ensures that it is able to capture and process early and late time ground responses. In the last decade the system has undergone a range of hardware and software developments with the aim of satisfying the changing landscape of geophysical exploration. Specific improvements have been made to improve the system’s ability to monitor and compensate for variations in system geometry. Inverse modelling and recent survey data are used to demonstrate the impact of system improvements on enhancing the system’s ability in a multitude of geophysical applications.
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Fast denoising of AEM data using Singular Value Decomposition
More LessAirborne Time-Domain ElectroMagnetic (TDEM) surveys are increasingly carried out in anthropized areas as part of environmental studies. In such areas, noise arises from either natural sources, such as spherics, or cultural sources, such as couplings with man-made installations. TDEM data may therefore be affected by many distortions, such as spikes, oscillations and shifts, which make the EM noise spectrum complex and may lead to erroneous inversion and subsequent misinterpretations. In such noisy environments, thresholding and stacking standard techniques, commonly used to filter TDEM data, are hardly efficient. Time-consuming and subjective manual cleaning of the data is therefore required. We propose an alternative fast and efficient user-assisted filtering approach. We adapted the Singular Value Decomposition (SVD) to denoise TDEM data. The SVD method uses the principal component analysis. It allows separating noise from geological signal extracting into components the dominant shapes from a series of raw input curves. The signal components are then used to reconstruct the EM decays without the noise. The SVD procedure was implemented in the denoising of several EM datasets acquired over anthropized areas in various contexts. The comparison between each reconstructed decay and its corresponding measured decay allows efficiently detecting noisy gates and rejecting mainly spikes and oscillations. Moreover, an ad hoc analysis of the map of weights of the components explaining noise showed high correlation with man-made installations. Thus, the SVD also provides a tool to reject most likely soundings biased by coupling noises, which may result in artefacts on the inverted models. However, some distorted decays can only be localized based on the analysis of specific SVD components. It was also shown that the maps of the weights of the components explaining the geological signal could be useful as a first rapid view of the contrasts that exist in subsurface. This established SVD based procedure is fast and provides accurate denoising tools; it makes, at least, the manual cleaning less time consuming and less subjective.
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A Bayesian Approach to the Inversion of Airborne Electromagnetic Data for a Multidimensional Earth
Authors Juerg Hauser, David Annetts and James GunningResistivity distributions for the subsurface based on airborne electromagnetic data are commonly derived using deterministic inversion methods. It is well known that this inverse problem is inherently non-unique; if one model can be found, it is likely that there exist alternative models that fit the data equally well, particularly once noise on the data is taken into account. Probabilistic approaches, like the one introduced in this work, allow exploration of the posterior distribution which represents the distribution of models that are in agreement with the data and the prior information. This work uses multipoint geostatistical models to represent the prior information, and a Markov Chain Monte Carlo technique to sample the unknown posterior distribution. The airborne electromagnetic data are predicted by employing a 2.5D forward solver, so that lateral changes in structure along the flight path are taken into account. The inversion aims at determining the probability of individual lithologies to be present. We use synthetic examples to demonstrate how the method recovers well-known facts of airborne electromagnetic imaging, for example the reduced resolution if targets are located under conductive regolith cover. Application of the method to field data collected over the Harmony Ni-S deposit in Western Australia shows that our sets of samples of the posterior distribution provides a more complete picture of solution space when compared to deterministic inversion results. Such probabilistic images of the subsurface can ultimately be beneficial for the mitigation of exploration risk, because of their quantification of uncertainties.
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Computationally efficient EM modelling using Shur decomposition applied to large scale airborne applications
Authors Trevor Irons and Yaoguo LiInversion of airborne electromagnetics problems can be extremely challenging due to the large number of transmitter locations. Differential-equation based solutions offer compelling advantages over integral equation and stitched 1D inversions, but come at a high computational cost and can struggle with the inclusion of the air layer. We present a new modelling scheme that is well-posed in the air, is extremely memory efficient. The algorithm is numerically attractive. Furthermore, preconditioners can be used to which allow for very quick convergence of iterative solvers. These preconditioners can be used across transmitter locations, source frequency, and conductivity model—making the algorithm well suited for AEM forward modelling and inversion on parallel architecture.
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Scalable, parallel constrained inversion of large AEM datasets
Authors C. Kirkegaard and E. AukenOver the past decade the typical size of AEM datasets has been growing rapidly, while at the same time targeting new applications that rely on advances in terms of resolution and accuracy. Approximate inversions and data transform techniques have previously defined the norm for interpretation of huge surveys, but rarely pose attractive solutions for modern applications such as aquifer mapping, uranium exploration, integrated modeling etc. For these applications high-resolution full system modeling techniques provide the only acceptable solution, but their refinement comes at the expense of significant added computational complexity. Further applying spatial constraints to the inversion of multiple 1D soundings facilitate a resulting quasi-3D model, however, there are severe intrinsic issues in effectively solving the underlying systems of linear equations. Here, we describe how we have attacked scalability issues of the Spatially Constrained Inversion (SCI) formalism and optimized our code to handle arbitrarily large problems on parallel multi-processor computers.
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Using the in-line component for fixed-wing EM layered inversion
More LessMany authors have discussed the utility of multicomponent measurements. Generally speaking, the vertical component couples to horizontal bodies while the in-line component couples best to vertical targets. For layered-earth cases, helicopter EM systems have little or no in-line component response and as a result much of the in-line signal is due to receiver coil rotation and appears as noise. In contrast to this, the in-line component of a fixed-wing airborne electromagnetic (AEM) system can be substantial, exceeding the vertical component in conductive areas. In this paper I compare the in-line and vertical response of a fixed-wing system to a halfspace and show the sensitivity functions. I then calculate the expected inversion model parameter uncertainty, showing that the use of both components better resolves model properties. I then compare inversion using both components to vertical component alone, providing an example using field data from the oil sands region of Canada.
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AEM Discrete Conductor Inversion
Authors M. A. Vallée, J. Lemieux and T. KimuraDiscrete object modelling has been used since the beginning of airborne electromagnetic exploration. A recent effort of integrating existing modelling and inversion tools with airborne data shows that simple models are still quite useful for exploration. They can provide simple parameters of a model with minimum a priori and still provide adequate data fitting. Approximations allow using models with reasonable computer resources. Plate models have been popular and inversion programs have been developed around them. Another inversion approach is based on approximation of sphere or plate response in free-space. Finally, a recent development is an analytic solution for a conductive sphere in a layered earth. We present some applications of this methodology.
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Accurate processing and inversion as the ultimate QC of AEM data
More Lesse ground gives higher signal, especially at early times/high frequencies , even over the same piece of ground. The readings in the Rx are the convolution of ground response and system transfer response (STF). In re-flights, what needs to be repeatable –hence precise- is the ground component part of the readings. The only way to assess precision is therefore to invert the data, uncoupling, from the measured signal, system STF and ground response. Another concept, rarely directly used in QC, but sometimes hinted at, and sometimes confused with precision, is that of accuracy. The latter is the measurement of closeness of measurement of a quantity to its actual (true) value. It should be evident that precision (repeatability) is not that useful per se, if the measured value we can repeat to a satisfying degree is not accurate (i.e., it is repeatedly far from the true value). An AEM system is well calibrated, if it is accurate. It is obvious that also in order to assess the accuracy (calibration) of an AEM system in rendering the actual distribution of the electrical resistivity, we need to work in the model space. The first approach is to invert the AEM data and compare the outcome with a relevant resistivity reference model (obtained from DC/borehole data, other EM data). The second is to forward model the reference resistivity model with the STF of the particular system in the condition of acquisition, and compare it with the actual observations of the system over it. For different reasons, the first approach tends to be the most tempting. It must however be stressed that an inversion output is the end result of a series of steps. From survey design, to data acquisition, pre-processing, post-processing, data integration (another issue that only makes sense contemplating the model space), and inversion, they all contribute to the output. Inaccurate processing and inversion can jeopardize the recovered models, and in turn also the ultimate assessment on AEM data precision and accuracy. If a problem of inaccuracy/poor calibration of an AEM system has been detected, it is again through inversions that, in some cases, the accuracy/calibration of that dataset might be improved. We will present examples that illustrate all the aspects mentioned above.
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The Nuqrah massive sulphide SEDEX deposit in Saudi Arabia - A Fugro TEMPEST perspective
Authors Darren Burrows, Reece van Buren and Emad Al SaadawiA TEMPEST airborne fixed wing Time Domain Electromagnetic (TDEM) and magnetic survey was flown over the Nuqrah – Mardah project area of central Saudi Arabia for Ma’aden Gold and Base Metals. The Nuqrah massive-sulphide sedex deposit is located within graphite schist and carbonates near the top of the pyroclastic Halaban formation. The survey has identified known mineral occurrences, and located new mineral targets. The north and south zones of the Nuqrah Deposit have been imaged, and further drilling may be indicated at Nuqrah North based on a strike length interpreted from CDI profiles of 1.6 km.
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Flying the time domain electromagnetic GENESIS system over the traditional Welkom gold mining district, Free State, South Africa, to identify contaminant seepage and acid mine drainage
By D. EberleIn May 2010 a time domain airborne electromagnetic (TD-AEM) survey was carried out for the Council for Geoscience over two selected areas in the traditional West Rand and Free State gold mining areas of South Africa. The goal of the TD-AEM survey was to acquire a 3D electric conductivity model of the survey areas reflecting the geological layering and potential pathways for rising acid mine waters or leaking contaminant fluids from tailings and slime dams. The TD-AEM Genesis system which was a fairly recent development mounted on a single engine Cessna Grand Caravan 208 was provided by Fugro Airborne Services (FAS). The survey was flown at 200 m flight line spacing, with flight lines oriented N-S and tie lines oriented W-E at 2000 m spacing. The EM transmitter and stinger mounted Caesium vapour magnetometer were flown at about 90 m terrain clearance. The EM receiver bird was closer to surface (45 m) and dragged by the aircraft. The transmitter-receiver (Tx-Rx) configuration is non-symmetric. Base frequency of the transmitter (Tx) was 75 Hz, peak moment 60300 Am2 and peak current 450 A. In high conductivity areas the depth of investigation of the Genesis system has been limited to little more than 100 m, only in low conductivity environment a maximum depth of investigation of about 300 m was achieved. Despite these constraints, the 3D conductivity voxels display the geological layering, highlight tectonic features (folding, faults) and identify individual conductive spots which are possibly associated with leaking tailings. As a whole, the usefulness of fast and cost-effective AEM surveying when tackling the issues of acid mine waters and contaminant seepage has successfully been proven. However, the use of a helicopter-borne AEM system may be preferred with regard to spatial power of resolution and system geometry.
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