Time-domain Electromagnetic Surveys Research Articles

Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution heliborne geophysics

Abstract. We conducted a multidisciplinary study at the watershed scale of an andesitic-type volcanic island in order to better characterize the hydrogeological functioning of aquifers and to better evaluate groundwater resources. A heliborne Time Domain ElectroMagnetic (TDEM) survey was conducted over Martinique in order to investigate underground volcanic structures and lithology, characterized by high lateral and vertical geological variability and resulting in a very high heterogeneity of their hydrogeological characteristics. Correlations were made on three adjacent watersheds between resistivity data along flight lines and geological and hydrogeological data from 51 boreholes and 24 springs, showing that the younger the formations, the higher their resistivity. Correlation between resistivity, geology and transmissivity data of three aquifers is attested: within the interval 10–100 ohm m and within a range of 1 to 5.5 Ma, the older the formation, the lower its resistivity, and the older the formation, the higher its transmissivity. Moreover, we demonstrate that the main geological structures lead to preferential flow circulations and that hydrogeological watersheds can differ from topographical watersheds. The consequence is that, even if the topographical watershed is small, underground flows from an adjacent watershed can add significant amounts of water to such a catchment. This effect is amplified when lava domes and their roots are situated upstream, as they present very high hydraulic conductivity leading to deep preferential groundwater flow circulations. We also reveal, unlike basaltic-type volcanic islands, that hydraulic conductivity increases with age in this andesitic-type volcanic island. This trend is interpreted as the consequence of tectonic fracturing associated with earthquakes in this subduction zone, related to andesitic volcanic islands. Finally, our approach allows characterization in detail of the hydrogeological functioning and identification of the properties of the main aquifer and aquitard units, leading to the proposition of a hydrogeological conceptual model at the watershed scale. This working scale seems particularly suitable due to the complexity of edifices, with heterogeneous geological formations presenting high lateral and vertical variability. Moreover, our study offers new guidelines for accurate correlations between resistivity, geology and hydraulic conductivity for volcanic islands. Finally, our results will also help stakeholders toward a better management of water resources.

Quantitative appraisal of Heliborne and ground-based Time Domain Electromagnetic surveys for uranium exploration – A case study from Rajasthan, India

Soda-metasomatised and metasediments hosted uranium deposits constitute 18% of the world?s uranium resources. In India, such deposits are identified along the albitite line in the environs of North Delhi Fold Belts in the state of Rajasthan. These are structurally controlled with association of albitisation and pyroxenisation of metasediments, metallic sulphides and carbonaceous phyllites. Uraninite is the dominant mineral phase along with copper, molybdenum and sulphides. The low resistivity of the fractures associated with metallic minerals produces a significant contrast with the host rock, which can be located with electromagnetic methods. High-resolution heliborne geophysical surveys comprising of VTEM, magnetic and gamma-ray spectrometric were conducted to identify uranium deposits in the vicinity of albitite zone. This paper demonstrates the results of heliborne and ground followup geophysical surveys to prioritize targets for uranium exploration. For the first time in India, a ground based Time Domain Electromagnetic survey employing Geonics EM37 system was conducted over one of the target areas delineated based on the VTEM surveys. Interpreted results from ground data correlate well with the spatial locations of the EM conductors delineated from heliborne surveys. Additionally modelling of heliborne and ground data produces comparable physical parameters. The results obtained are extremely useful in planning the boreholes in ongoing exploration programme.

Airborne electromagnetic signature of a paleochannel uranium deposit

SUMMARY Reptile Uranium, a wholly owned subsidiary of Deep Yellow Ltd., has been actively exploring four exclusive prospecting licenses (EPLs) in Namibia since 2007. Prior exploration of this area in the 1970’s and 80’s resulted in the discovery of extensive Tertiary uranium mineralization hosted by paleochannels. Detailed airborne radiometric data acquired in 2007 shows good correlation with known surficial mineralization, but provides no depth information about the known channels, and does not identify potential buried channels. In 2008, Aeroquest Surveys completed an AeroTEM helicopter time domain electromagnetic (HTEM) survey over the four EPLs. The electromagnetic (EM) data proved to be an excellent tool for mapping the conductive material associated with both near-surface and buried paleochannels. There is generally good correlation between the stronger AeroTEM responses and the known mineralization. The HTEM data were inverted using a 1D layered earth algorithm to model the distribution of conductivity laterally and with depth. The modeled data effectively describe the lateral extent, thickness and relative conductivity of the paleochannels and have been found to be very useful in directing the ongoing drill program both for the delineation of known paleochannels and for the drilling of previously unknown paleochannels. Successful trenching and drill results from the Tubas-Oryx-Tumas paleochannel system clearly illustrate the important role that the HTEM data has played in the ongoing exploration on this project.

Method Integration Improves Reservoir-Property Prediction in East Siberia

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191673-18RPTC, “Seismic- and Electromagnetic-Methods Integration To Increase Quality of Reservoir-Property and -Saturation Prediction in East Sibera Region,” by Pavel Mostovoy, Roman Oshmarin, and Andrey Ostankov, GazpromNeft, and Olga Tokareva and Daria Orlova, IERP, prepared for the 2018 SPE Russian Petroleum Technology Conference, Moscow, 15–17 October. The paper has not been peer reviewed. This paper describes a case study of the Chonskaya group of fields to demonstrate an approach to the integration of time-domain-electromagnetic (TDEM) electrical and seismic data when building a geological model for improving reservoir-property and -saturation prediction. Introduction The terrigenous sediments of the subsalt complex within the Nepa-Botuoba anteclise are characterized by complex geology: nonanticlinal traps, complex tectonics, lateral reservoir heterogeneity, and digenesis that controls reservoir distribution. The block structure of the Chonskaya group of fields and their poor coverage with exploration wells (Fig. 1) leave a number of uncertainties with regard to water/oil and gas/water contacts in blocks with proved oil content, as well as the main fluid types in exploration blocks. The success of predicting prospective sites for exploration drilling is influenced by minimal risks and reduced uncertainty during field development. An important task at the stage of exploration drilling is to increase the reliability of fluid contacts and reservoir properties. To improve the quality of geophysical predictions during exploration of the Chonskaya group of fields, geological exploration operations were conducted, including 3D seismic surveys and high-density electrical studies [time-domain electromagnetic and magnetic (TDEM)] on a single observation network. The area studied by TDEM and seismic surveys on the observation network in the Chonskaya group covers 3500 km2, more than 50% of the total territory of the license areas. In this regard, the methods to honor the TDEM data when building and updating complex geological models of reservoirs are being actively developed, because the TDEM signal can provide information about fluids saturating terrigenous reservoirs. To understand the capabilities of the technology for integrating seismic and electrical data and making a decision on the basis of the TDEM survey, forward modeling is necessary. On the basis of the modeling results and experience in building complex geological models, the combined use of electrical and seismic data has the potential to be used to separate pure-oil, water/oil, and pure-water zones; to predict probable fluid types in the exploration targets; and to outline zones with poor reservoir properties in terrigenous rocks of the subsalt complex within the Nepa-Botuoba anteclise. A geological model honors the TDEM data through building a geoelectrical reservoir model using a priori geological and geophysical information about target reservoirs—the kinematic and dynamic interpretation of seismic data, well-log-interpretation data, a petrophysical model, and a geological model of the reservoir (2D or 3D). Next, a comparative analysis of the synthetic geoelectrical model with the TDEM inversion data is conducted. The geological model assumptions are revised on the basis of qualitative and quantitative comparison. The revised assumptions allow reduction of the range of geological uncertainties and adjustment of the geological exploration program.

Using representative synthetic data to analyze effects of filters when processing full waveform airborne TEM data

Airborne time domain electromagnetic (ATEM) surveys have reached the stage where full waveform streamed data are recorded and delivered in addition to traditional survey products. One result of this advance in technology is that the line between the acquisition and processing phases has become more flexible and many parameters that used to be hardwired in acquisition can now be adapted during the processing phase. In order to make use of this opportunity the interpreter needs a clear description and understanding of the system specific corrections required to isolate geological responses as well as the effects of filters and other digital signal enhancement options that are available. Validating procedures on a synthetic data set is one way of ensuring that all geological responses falling within similar parameter ranges would be accurately presented after processing. In this study the effects of three time-series and four spatial filters were analyzed. Streamed full waveform data were simulated by adding measured high altitude data to synthetic models. The various filters were applied and the deviations from the true models compared with that of the unfiltered data. The results were evaluated based on whether the filtered results showed more or less deviation than the unfiltered data from the original noise-free models.

Characterizing the Spiritwood Valley Aquifer, North Dakota, using helicopter time-domain EM

Buried valley aquifers, consisting of permeable sand and gravel deposits in eroded bedrock valleys, are important sources of groundwater supply in many regions of the United States and Canada. Investigations of the Spiritwood aquifer in southern Manitoba by the Geological Survey of Canada and other workers, have demonstrated the value of helicopter time domain electromagnetic (HTEM) surveys in aquifer mapping and characterization using the contrasts between Quaternary glacio-lacustrine sand-gravels (high resistivity) that are relatively permeable and clay-tills (low resistivity) that are relatively impermeable, as well as the deeper, much less resistive Cretaceous Pierre Formation Shale basement rocks. This success provided the impetus for the North Dakota State Water Commission to fly a VTEM helicopter EM survey in the Jamestown, ND region in October, 2016. The VTEM data collected over the Spiritwood-JT block allowed for geological mapping from near surface to depth, in spite of relatively weak resistivity contrasts (<10X). These data were inverted with a layered-earth algorithm to produce resistivity-depth models. These models were able to resolve the location and depths to the top and bottom of the Spiritwood aquifer throughout the central portion of the block providing more detailed pictures of the aquifer’s geometry. In addition to resolving the main aquifer as well as its deeper channels, the VTEM data and models highlighted several smaller, previously undiscovered aquifers that cross-cut/branch-off from the main Spiritwood channel. These are interpreted as probable transverse low-K barriers that were apparent from the existing test drilling and aquifer testing.

Heliborne Time Domain Electromagnetic Data Aiding Delineation of Carbonaceous Shale in Southwestern Part of Bijawar Basin and its Implication on Basinal Stratigraphy

Geophysical methods are tools to unravel concealed geological features that enhance knowledge. Heliborne time domain electromagnetic (TDEM) survey carried out over volcano-sedimentary sequence of Palaeoproterozoic Bijawar basin indicated presence of conductor in the western part of the basin. Subsequently boreholes drilled intercepted rhythmites made up of carbonaceous shale and dolomite within Bajno dolomite Formation. Measurement of conductivity on core samples established the high conductive carbonaceous shale as the contributor for TDEM response. TDEM signature indicate widespread presence of rhythmites in the western part of Bijawar basin reflecting different hydrodynamic condition from other parts of Bajno dolomite. Thus, Bajno dolomite Formation is classified into two members viz. Tigoda member and Sarwa member.

Geophysical Signature of the Southern Gurubang Base Metal Occurrence in South Eastern NSW

Ground-based, time-domain electromagnetic, magnetic and gravity datasets were obtained for the southern-section of the Gurubang VHMS deposit located approximately 15 km east of Cooma, NSW. The Gurubang deposit is hosted in a mid-late Silurian sequence of rock composed of shallow marine sediments and felsic volcanic rocks. The aim of this research was to ascertain the usefulness of high-resolution geophysical techniques in targeting and evaluating a small-scale polymetallic massive sulphide deposit, and to investigate how the detailed geophysics relates to the overall geological framework of the prospect area. The acquired data was analysed using a forward modelling approach. Due to the deposits high concentration of conductive minerals, a coincident loop time-domain electromagnetic 2D survey effectively delineated the sulphide mineralisation, and was useful in interpreting and adapting deposit parameters such as the azimuth, dip and strike length. Based on the physical nature of the target deposit, it was determined that high-resolution magnetic and gravity surveys would not be effective methods in directly delineating these smaller-scaled (10’s of m’s) mineral deposits. However, magnetics and gravity did prove effective in depicting the surrounding geology, including potential volcanic intrusions and basement lithologies and structures.

Time‐domain electromagnetic imaging of a clayey confining bed in a brackish environment: A case study in the Kairouan Plain Aquifer (Kelbia salt lake, Tunisia)

AbstractThe semi‐arid Plio‐Quaternary aquifer of the Kairouan Plain is located in the central part of Tunisia. In this region, the main form of human activity is extensive agriculture. The few lithological logs in this area indicate that the aquifer is divided into two groundwater reservoirs separated by a confining clay layer. In the shallower groundwater reservoir, the electrical conductivity of the water is close to 1,200 mS/m, that is, 3 to 4 times higher than that of the deeper aquifer, which is currently used for irrigation. Recently, an increase in salinity has been observed, which is producing significant difficulties with irrigation. In order to determine the geometry of the confining clay layer and assess the potential connectivity between both reservoirs, time‐domain electromagnetic (TDEM) and vertical electrical soundings surveys were carried out. In the extremely conductive environment (electrical resistivities &lt;10 Ωm) of the Sebkha Kelbia, the TDEM survey produced the first geophysical map of this near‐surface aquifer system.

Geophysical investigations in the Gañuelas-Mazarrón Tertiary basin (SE Spain): A natural analogue of a geological CO2 storage affected by anthropogenic leakages

In the framework of a Spanish project focused on CCS technologies (PSE-CO2 Project), the Gañuelas-Mazarrón Tertiary Basin (SE Spain) was globally studied as a natural analogue of a CO2 reservoir affected by anthropogenic leakages. In this context, geophysical investigations were carried out in order to determine the location, morphology and extent of the CO2-enriched saline aquifer existing in the bottom of the basin, and evidenced by two core drills performed in the 1980s for geothermal purposes (La Ermita de El Saladillo and El Alto de El Reventón. The geophysical researches were performed by reprocessing the vertical electrical soundings also made in the 1980s, but also by means of new campaigns of electrical resistivity tomography, time-domain electromagnetic surveys and gravimetry. The main results from geoelectric and electromagnetic investigations have allowed identifying three main different geological materials in the basin based on their resistivity: metamorphic materials from the Nevado-Filábride Complex, and Tertiary sedimentary materials mainly consisted of sandy calc-marls and intruded by more or less altered Neogene volcanic rocks. Furthermore, it has been defined the basin as a graben limited by the Nevado-Filábride materials affected by very vertical faults and frequently filled by volcanic rocks, reaching a depth of about 500 m. However, the gravimetric research has allowed to better identifying the strong gradients in the borders of the basin, which is indicative of fairly net and vertical contacts, and also to establish a geological model of the basin. This model includes the SW and NE boundaries very vertical and a deeply sunken central zone with about 9 km wide. This area has been divided in turn into two sections, within which the one located at the SW (~5 km) has a depocenter that reaches a depth of about 900 m. Consequently, the possibility that the CO2 be in supercritical state in certain areas of the aquifer, which should be hosted at the contact between the Tertiary sediments/volcanic rocks and the Nevado-Filábride materials, has not to be discarded. Therefore the studied basin would represent an excellent natural analogue for a geological CO2 storage in a deep saline aquifer.

Coupled simulation of electromagnetic induction and induced polarization effects using stretched exponential relaxation

We have developed a new algorithm for 3D time-domain electromagnetic (EM) modeling, taking full account of induced polarization (IP) and the coupling between EM and IP effects. The algorithm can be used to model grounded source IP surveys that indicate EM induction effects and airborne time-domain EM surveys that exhibit IP effects. IP effects are most often approximated as static or modeled in the frequency domain, using frequency-dependent electrical conductivity. It is difficult to translate the frequency-dependent conductivity approach directly to the time domain in a computationally efficient manner. We take an alternative approach in which we model IP relaxations in time using the stretched exponential (SE) function. We incorporate this IP model into a direct time-stepping discretization of the quasistatic time-domain Maxwell equations. We found that modeling of IP effects with this SE approach is asymptotically equivalent to the commonly used Cole-Cole model of IP transformed to the time domain. We have implemented our algorithm using efficient numerical methods that allow it to tackle large-scale problems and are amenable to use in inversion. In particular, we have developed a parallel time-stepping technique that allows us to compute transient electric fields at multiple time steps simultaneously. We demonstrate the behavior of the SE model of IP decay and the efficiency of our algorithm by applying it to synthetic numerical examples that simulate a grounded source IP survey with significant EM effects and a concentric-loop airborne EM sounding over a chargeable body.

Characterizing leachate contamination in a landfill site using Time Domain Electromagnetic (TDEM) imaging

Time Domain Electromagnetic (TDEM) survey as well as drilling investigations were conducted to identify possible contamination of a dumping site in an unsaturated zone located in Barka, Oman. The method was applied to evaluate conductivity of the contaminated plumes in hot and arid/semiarid region, where high temperatures commonly ranged between 35 and 50 °C. The drilling investigation was carried out over the survey area to verify the geophysical results. The low-resistivity zone (<80 Ωm), encountered near the subsurface, indicated plume migration caused by liquid waste disposal activities. The combination of TDEM survey results with the lithology of piezometers showed that higher resistivity (>90 Ωm) was correlated with compacted or cemented gravels and cobbles, particularly that of medium dense to very dense gravels and cobbles. Additionally, the TDEM profiles suggested that the plume migration followed a preferential flow path. The resistivity range 40–80 Ωm considered as contaminated areas; however, the drilling results showed the close resistivity domain in the depth >70 m below water table for some profiles (BL1, BL2, BL3, BL4 and BL5). The combined results of drilling wells, piezometers, and TDEM apparent resistivity maps showed a coincidence of the migrated leachate plume and water table. Predicted zone of the probable contamination was located at the depth of around 65 m and horizontal offset ranges 0–280 m, 80–240 m, and 40–85 m in the sounding traverses of BL4, BL6 and BL7, respectively.

Inversion of airborne geophysics over the DO-27/DO-18 kimberlites — Part 3: Induced polarization

The geologically distinct DO-27 and DO-18 kimberlites, often called the Tli Kwi Cho (TKC) kimberlites, have been used as a testbed for airborne geophysical methods applied to kimberlite exploration. This paper focuses on extracting chargeability information from time-domain electromagnetic (TEM) data. Three different TEM surveys, having similar coincident-loop geometry, have been carried out over TKC. Each records negative transients over the main kimberlite units and this is a signature of induced polarization (IP) effects. By applying a TEM-IP inversion workflow to a versatile time domain EM (VTEM) data set we decouple the EM and IP responses in the observations and then recover 3D pseudo-chargeability models at multiple times. A subsequent analysis is used to recover Cole-Cole parameters. Our models demonstrate that both DO-18 and DO-27 pipes are chargeable, but they have different Cole-Cole time constants: 110 and 1160 μs, respectively. At DO-27, we also distinguish between two adjacent kimberlite units based on their respective Cole-Cole time constants. Our chargeability models are combined with the density, magnetic susceptibility and conductivity models to build a 3D petrophysical model of TKC using only information obtained from airborne geophysics. Comparison of this final petrophysical model to a 3D geological model derived from the extensive drilling program demonstrates that we can characterize the three main kimberlite units at TKC: HK, VK, and PK in three dimensions by using airborne geophysics.

Assessment of background gamma radiation levels using airborne gamma ray spectrometer data over uranium deposits, Cuddapah Basin, India – A comparative study of dose rates estimated by AGRS and PGRS

The Atomic Minerals Directorate for Exploration and Research (AMD) has conducted high-resolution airborne gamma ray spectrometer (AGRS), magnetometer and time domain electromagnetic (TDEM) surveys for uranium exploration, along the northern margins of Cuddapah Basin. The survey area includes well known uranium deposits such as Lambapur-Peddagattu, Chitrial and Koppunuru. The AGRS data collected for uranium exploration is utilised for estimating the average absorbed rates in air due to radio-elemental (potassium in %, uranium and thorium in ppm) distribution over these known deposit areas. Further, portable gamma ray spectrometer (PGRS) was used to acquire data over two nearby locations one from Lambapur deposit, and the other from known anomalous zone and subsequently average gamma dose rates were estimated. Representative in-situ rock samples were also collected from these two areas and subjected to radio-elemental concentration analysis by gamma ray spectrometer (GRS) in the laboratory and then dose rates were estimated. Analyses of these three sets of results complement one another, thereby providing a comprehensive picture of the radiation environment over these deposits. The average absorbed area wise dose rate level is estimated to be 130 ± 47 nGy h−1 in Lambapur-Peddagattu, 186 ± 77 nGy h−1 in Chitrial and 63 ± 22 nGy h−1 in Koppunuru. The obtained average dose levels are found to be higher than the world average value of 54 nGy h−1. The gamma absorbed dose rates in nGy h−1 were converted to annual effective dose rates in mSv y−1 as proposed by the United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR). The annual average effective dose rates for the entire surveyed area is 0.12 mSv y−1, which is much lower than the recommended limit of 1 mSv y−1 by International Commission on Radiation protection (ICRP). It may be ascertained here that the present study establishes a reference data set (baseline) in these areas to assess any changes in gamma radiation levels due to mining and milling activities in future.

Mapping subtrappean sediments and delineating structure with the aid of heliborne time domain electromagnetics: Case study from Kaladgi Basin, Karnataka

Mapping of subtrappean sediments is a complex geological problem attempted by many interpreters applying different geophysical techniques. Variations in thickness and resistivity of traps and underlying sediments, respectively, results in considerable uncertainty in the interpretation of geophysical data. It is proposed that the transient electromagnetic technique is an effective geophysical tool for delineation of the sub-trappean sediments, due to marked resistivity contrast between the Deccan trap, and underlying sediments and/or basement. The northern margin of the Kaladgi basin is covered under trap.A heliborne time domain electromagnetic survey was conducted to demarcate the basin extent and map the sub-trappean sediments.Conductivity depth transformations were used to map the interface between conductive trap and resistive ‘basement’. Two resistivity contrast boundaries are picked: the first corresponds to the bottom of the shallow conductive unit interpreted as the base of the Deccan Volcanics and the second - picked at the base of a deeper subsurface conductive zone – is interpreted as the weathered paleo-surface of the crystalline basement.This second boundary can only be seen in areas where the volcanics are thin or absent, suggesting that the volcanics are masking the EM signal preventing deeper penetration. An interesting feature, which shows prominently in the EM data but less clearly imaged in the magnetic data, is observed in the vicinity of Mudhol. The surface geology interpreted from satellite imagery show Deccan trap cover around Mudhol. Modelling of TDEM data suggest the presence of synclinal basin structure. The depth of penetration of the heliborne TDEM data is estimated to be approximately 350m for the study area. This suggests that heliborne TDEM could penetrate significant thicknesses of conductive Deccan trap cover to delineate structure below in the Bagalkot Group.

Comparing induced polarization responses from airborne inductive and galvanic ground systems: Lewis Ponds, New South Wales

We have evaluated the mapping of polarizable material using a Cole-Cole model to fit second-order effects in concentric-loop airborne electromagnetic system responses. At Lewis Ponds in New South Wales, an inverted ground dipole-dipole array data has accurately imaged in 3D disseminated sulfide extending above and around ore grade massive sulfides. The polarizable zone is present in the near-surface, where, from modeling, airborne systems may have sensitivity to the small inductive induced polarization effects. Although the inverted chargeability measured from galvanic current injection into the ground was spatially coincident with the mineralized target, the estimated chargeabilities from induced polarization effects in an airborne versatile time-domain electromagnetic survey were substantially displaced to the east. The airborne induced polarization response may be associated with finer grained mineralization in the hanging wall of the sulfide deposits, or have a quite different source, such as clays associated with faulting.

The Camelwood and Musket Nickel deposits - Discovery of a new Nickel Sulphide camp in the North-eastern Goldfields of Western Australia

The Camelwood and Musket nickel sulphide deposits are significant recent discoveries, located within the Mt Fisher Greenstone Belt, in the northern goldfields region of Western Australia. Camelwood was the first deposit to be discovered, in December 2012, from a reverse circulation (RC) drilling campaign designed to test a coincident airborne electromagnetic (AEM) and geochemical anomaly. The original objective of the AEM surveys was to detect massive sulphides known to be associated with gold mineralization at the old Mt Fisher gold mine. However, a number of discrete, late-time EM anomalies were identified along an interpreted ultramafic sequence on the eastern boundary of the greenstone belt. The EM anomalies represented classic nickel sulphide mineralisation targets. Ground time-domain electromagnetic (TEM) surveys, down-hole TEM (DHTEM) surveys, and extensive drilling have been carried out since then, resulting in a JORC compliant resource at Camelwood (1.6Mt @ 2.2% Ni) and the discovery of the Musket deposit. The application of the AEM method was instrumental in the discovery of the Camelwood nickel deposit. Systematic use of ground and down hole geophysical methods has been valuable in delineating the resource at Camelwood and in the discovery of the Musket deposit. The discovery of Camelwood and Musket proves the potential of the Mt Fisher Greenstone belt to host significant nickel sulphide mineralisation.

A New Apparent Resistivity Formula for In-loop Fast Sounding TEM Theory and Application

The central-loop time-domain electromagnetic (TEM) method has been widely used in hydrogeological prospecting. Because this type of TEM configuration is relatively insensitive to lateral resistivity contrasts, the mathematical derivation of an apparent resistivity formula, valid for later recording times, is greatly simplified. However, no simple formula exists for an in-loop off-center TEM sounding. In this type of acquisition, the field response is recorded at several points surrounding the central location. In-loop TEM surveys are frequently employed to obtain high spatial resolution within engineering geophysical exploration. Although the apparent resistivity formula of a fixed-loop TEM system is also applicable for the non-central receiver points used in an in-loop TEM system, the apparent resistivity values need to be extracted using an iterative method that requires complicated integrations. Presently, no fast method exists that can give access to the apparent resistivity values in a direct manner. The main objective of this paper is to present a possible solution to this computational problem. A new solution is presented that takes as a starting point a circular transmitter loop where an analytical solution exists for non-central receiver points. This analytical solution is further approximated through the combined use of least-mean square (LSQR) determined polynomial coefficients and an equivalent circular loop to represent the rectangular loop. The new apparent resistivity formula, valid for in-loop TEM, has been tested on field data. A successful case study from central China is presented where an increased sensitivity to locate water-filled zones has been obtained.

ZTEM and VTEM airborne EM and magnetic results over the Lalor copper‐gold volcanogenic massive sulfide deposit region, near Snow Lake, Manitoba

The 14.4-Mt Lalor copper-zinc-gold deposit situated in the Flin Flon Greenstone Belt of north-central Manitoba is large ([Formula: see text]) but deeply buried ([Formula: see text]) and was discovered using deep penetrating ground fixed-loop time-domain electromagnetic (EM). Results from three different airborne EM surveys over a five-year span permit comparisons of the system responses over the Lalor volcanogenic massive sulfide deposit region, assisted using 1D-2D-3D inversions. In late March, 2007, soon after its drilling discovery, Hudson Bay Exploration contracted Geotech to fly helicopter versatile time-domain electromagnetic (VTEM) surveys over the region west of Snow Lake, Manitoba. The deposit was not detected due to its large depth of burial ([Formula: see text]) and the lack of magnetic response. In 2009, Geotech carried out a helicopter z‐axis tipper electromagnetic (ZTEM) passive EM test survey over the Lalor deposit, which was successful and led to a larger survey over the area. In 2012, a second VTEM survey test was flown, using the more deeply penetrating [Formula: see text] system. Although it had sufficient penetration to detect the orebody, on-site mine-site development appeared to overprint the deeper [Formula: see text] deposit response. However, a deep ([Formula: see text]) conductive anomaly from a stringer sulfide occurrence, known as South Bull’s Eye, which lies south of Lalor, was well resolved in the ZTEM and [Formula: see text] surveys.

Use of a time-domain electromagnetic method with geochemical tracers to explore the salinity anomalies in a small coastal aquifer in north-eastern Tunisia

The study area is a small coastal plain in north-eastern Tunisia. It is drained by an ephemeral stream network and is subject to several pollutant discharges such as oilfield brine coming from a neighboring oil company and wastewater from Somâa city, located in the upstream of the plain. Furthermore, a hydraulic head near the coastal part of the aquifer is below sea level, suggesting that seawater intrusion may occur. A time-domain electromagnetic (TDEM) survey, based on 28 soundings, was conducted in Wadi Al Ayn and Daroufa plains to delineate the saline groundwater. Based on longitudinal and transversal resistivity two-dimensional pseudosections calibrated with boring data, the extent of saline water was identified. Geochemical tracers were combined with the resistivity dataset to differentiate the origin of groundwater salinization. In the upstream part of the plain, the infiltration of oilfield brine through the sandy bed of Wadi Al Ayn seems to have a considerable effect on groundwater salinization. However, in the coastal part of the aquifer, groundwater salinization is due to seawater intrusion and the saltwater is reaching an inland extent around 1.3 km from the shoreline. The contribution ratios of saline water bodies derived from the inverted chloride data vary for the oilfield brine from 1 to 13 % and for the seawater from 2 to 21 %.