Time-domain Electromagnetic Surveys Research Articles

The application of airborne geophysics in the exploration for iron ore in the Zouerate area of Mauritania

An airborne time domain electromagnetic and magnetic survey (Geotech VTEM system; VTEM is a helicopter-borne Time Domain Electromagnetic (TDEM) system with concentric loop transmitter receiver geometry.) was flown over the Kedia mountain and Mhaoudat Intrusive in Mauritania. Hematite has been mined in both these areas for many years. The objectives of this survey were to determine if additional ore bodies existed below old mined areas and to find and delineate possible new ore bodies. A detailed structural interpretation, using the magnetic and EM data, was done with the assumption that major faults could be a controlling factor on the occurrence of iron ore. Decay curve analysis was done of the VTEM data as a fast, first pass delineation of conductive areas. Good correlation with existing mines was found. CDI?s were consequently calculated for all flight lines. These were used to calculate conductivity contour maps at various depth intervals below the surface. Conductive areas were correlated with the magnetic data as well as the known geology. In several areas known iron ore was associated with the edges of observed anomalies. These anomalies continued at depth and distance, indicating possible extensions of the ore. Further anomalies occurred in a sand covered area south of Mhaoudat, leading to the possibility of new iron ore deposits.

Resistivity structure of Unzen Volcano derived from time domain electromagnetic (TDEM) survey

Time-domain electromagnetic (TDEM) surveys were carried out around Unzen Volcano, Shimabara Peninsula, South-west Japan in 2001 and 2002 in the eastern part of the peninsula. The surveys were a completion of the previously interpreted TDEM data collected from surveys in 1995 in the western part of the peninsula. Interpretation of the recorded transient magnetic fields resulted in a general feature of the resistivity structure which are composed of three main layers: a resistive surface layer, a conductive layer (below 10 Ωm) from about sea level to 2.5 km below the surface, and a third resistive layer. The conductive layer is considered to be a complex of water-saturated and altered layers which spreads widely beneath Shimabara Peninsula. The spatial conductance distribution shows a W–E trending pattern of high conductance from Tachibana Bay to the summit of Unzen (Fugen–Dake). This evidence indicates that volcanic gas was supplied to shallow layer in the western part of the peninsula, which is in accordance with the path of magma migration derived from seismic and deformation studies. Another high conductance zone was also found near Mayu–Yama in the eastern part of Shimabara Peninsula, indicating magma degassing process due to the possibility of the presence of a magma system beneath it.

Electromagnetic Soundings of the Regolith at Kalkaroo Mineral Prospect, Curnamona Province, South Australia

Large parts of Australia are blanketed with a thick regolith that masks mineral deposits. Time Domain Electromagnetic (TEM) technique is a proven method for imaging structures below or within the weathered overburden. Four NW-SE SIROTEM transect lines (three 1.5 km long and one 2.5 km long) were conducted over a magnetic anomaly in the Cu-Au Kalkaroo prospect in the Curnamona Province (S.A.). The TEM survey geometry used 100 metre square transmitting loops with an in-loop central receiver coil and 100 metre station spacing to maximise penetration into the electrically conductive overburden. Initial results of the survey, following a ``STEMINV' smooth model inversion process, suggest the TEM soundings penetrate to a depth of 100-150 m and reveal the presence of conductive regolith overlying a resistive zone. Furthermore, the 2 D depth-resistivity image obtained for all the TEM transects clearly indicates the presence of a highly conductive layer within the regolith at a depth of about 20-50 m. Through correlation with nearby drillhole data and potential field data, this conductive zone is interpreted to be the Namba Formation. The TEM results also indicate some of the structural features of the basement.

The relationship between saline groundwater within the Arava Rift Valley in Israel and the present and ancient base levels as detected by deep geoelectromagnetic soundings

The time domain electromagnetic (TDEM) geophysical method was employed to detect saline groundwater bodies within and in the close margins of the Arava Rift Valley. The Arava Valley aquifers are known to occupy fresh to saline groundwater. The lateral subsurface inflow to the Arava from west and east is characterized by fresh to brackish waters. The results of the present study indicate that salination of groundwater is controlled by both present day and ancient base levels, namely by the Dead Sea in the north and by the Gulf of Elat in the south. The configuration obtained by the TDEM survey exhibits interfaces and palaeo-interfaces between fresh to brackish waters and underlying seawater or diluted seawater intruded inland from both base levels as well as brines intruded from the northern base level. The central Arava structural and hydrological divide seems to escape seawater or brine encroachment at least to the considerable depth of the TDEM measurements.

Application of time-domain airborne electromagnetic induction to hydrogeologic investigations on the Pajarito Plateau, New Mexico, USA

We conducted a time-domain airborne electromagnetic (AEM) survey of part of the semiarid Pajarito Plateau of northern New Mexico to determine depths and lateral extent of perched aquifers in the vadose zone and depths and pathways of infiltration to the regional aquifer. The electrical resistivity of the plateau ranged over three orders of magnitude ([Formula: see text] to [Formula: see text]) to a depth of at least [Formula: see text]. Borehole and surface-derived data allow the correlation of resistivity images with the hydrogeology of the plateau. As expected, water exerts a significant control on resistivity. However, the presence of large amounts (up to 90%) of clay in some units, in conjunction with water, also has a significant effect, lowering resistivity (to [Formula: see text]) more than the presence of clay-free saturated zones alone. Because of the resulting low resistivity, we are able to better delineate a large,known volume of clay-altered volcaniclastic rock and postulate the presence of another. Resistivity values of [Formula: see text] cor-relate with depths to saturated zones where no clay is present, but they do not allow us to distinguish between one large or several smaller perched groundwater zones and the underlying regional zone of saturation. We imaged a region of significant infiltration related to a sewage treatment plant and to near-surface hydrogeo-logic conditions conducive to infiltration and correlated with a region of preferential transport of anthropogenic chemicals through the vadose zone. AEM data provide an important synop-tic view of the shallow (few hundred meters) resistivity structure of the plateau. Although interpretation of the data is not unique, when combined with borehole geologic, hydrologic, and geo-chemical data, it can provide relative depths to saturated zones, delineate regions of high clay content (zones of alteration), and image regions of recharge to the regional aquifer.

Time-Domain Electromagnetic Data Interpretation using Moving-Loop Configurations for Sheet-Like Base Metal Ore Deposits in Resistive Hosts

A simple, useful, and practical tool is proposed for the interpretation of moving-loop time-domain electromagnetic (TEM) surveys over two-dimensional (2D) sheet-like conductors embedded in a resistive host. It is based on the relationship observed between attributes of the displayed responses and the geometrical and electrical parameters of the conductive ore body. The ore body is modelled as a plate conductor for which the depth, dip, and conductance parameters are estimated. Numerical and scale modelling are used to establish the interpretative expressions. Responses computed for the various plate parameters are classed according to the following response attributes: time constant, asymmetry, and peak-to-peak distance. Three expressions relating depth, dip, and conductance to the response attributes are determined using multiple linear regression. The relationships are validated using scale-modelling data. The method allows the determination of the plate depth, conductance, and dip with an accuracy of ±10%, ±10% and ±5° respectively. The method is tested on SIROTEM survey data from Chutes-des-Passes in Quebec (Canada), where drill hole information is available. The results show that the regression relationships provide accurate estimates of the basic characteristics of the deposits.

Architecture and evolution of a fjord‐head delta, western Vancouver Island, British Columbia

AbstractThe architectural framework and Holocene evolution of the Zeballos fjord‐head delta on west‐central Vancouver Island was established through a multidisciplinary field‐based study. The Zeballos delta is a composite feature, consisting of an elevated, incised, late Pleistocene delta and an inset Holocene delta graded to present sea level. Both deltas have a classic Gilbert‐type tripartite architecture, with nearly flat topset and bottomset units and an inclined foreset unit. Time domain electromagnetic (TDEM) and ground‐penetrating radar (GPR) surveys, borehole data, and gravel pit exposures provided information on the internal form, lithologies and substrate of both deltas. Both sets of deltaic deposits coarsen upward from silt in the bottomset unit to gravel in the topset unit. The TDEM survey revealed a highly irregular, buried bedrock surface, ranging from 20 m to 190 m in depth, and it delineated saltwater intrusion into the deltaic sediments.Late Quaternary sea‐level change at Zeballos was inferred from delta morphology and the GPR survey. The elevated, late Pleistocene delta was constructed when the sea was about 21 m higher relative to the land than it is today. It was dissected when sea‐level fell rapidly as a result of glacio‐isostatic rebound. Relative sea‐level reached a position about 20 m below the present datum during the early Holocene. Foreset beds that overlap and progressively climb in a seaward direction and topset beds that thicken to 26 m landward imply that the delta aggraded and prograded into Zeballos Inlet during the middle and late Holocene transgression. Sea‐level may have risen above the present datum during the middle Holocene, creating a delta plain at about 4 m a.s.l. Remnants of this surface are preserved along the valley margins. Copyright © 2004 John Wiley & Sons, Ltd.

Geophysical monitoring and evaluation of coastal plain aquifers.

We use time domain electromagnetic (TDEM) soundings to monitor ground water conditions beneath the coastal plain in eastern North Carolina. The TDEM method measures the earth's response to an induced electromagnetic field. The resulting signal is converted, through a complex inversion process, to apparent resistivity values, which can be directly correlated to borehole resistivity logs. TDEM soundings are used to map the interface between fresh and salt water within coastal aquifers, and estimate depth to basement when siting new monitoring wells. Focused TDEM surveys have identified areas of salt water encroachment caused by high volumes of discharge from local supply wells. Electromagnetic sounding, when used in tandem with the state's network of monitoring wells, is an accurate and inexpensive tool for evaluating fresh water/salt water relationships on both local and regional scales within coastal plain aquifers.

米国カリフォルニア州ロストヒルズ油田で実施した，坑井間電磁および弾性波トモグラフィーによる貯留層モニタリングの試み

Reservoir monitoring is the key issue to design the optimum plan for recovery enhancements of hydrocarbon production. The Technology Research Center of Japan National Oil Corporation has conducted extensive field measurements to validate the current cross-hole technologies at the Lost Hills Oil Field, California in the first quarter of 2000. The experiments were composed of cross-hole electromagnetic surveys between fiberglass cased boreholes in both frequency domain and time domain methods together with seismic tomography. The time domain electromagnetic survey was designed to test the newly developed time domain down-hole transducer. However, the tool was broken down during the measurement. For the frequency domain electromagnetic method, we conducted three kinds of experiments, which are time lapse analysis with previously measured data, multi-frequency survey for higher resolution analysis, and measurement with setting up transducer in a steel cased hole. Multi-stage multi-disk borehole seismic source, which has been developed in collaboration between JNOC and OYO corporation was employed for seismic tomography.The subsurface velocity profile from seismic tomog raphy was utilized to calculate the formation porosity. Since the standard calculation procedure of sonic porosity is not adequate to apply for the diatomite reservoir because of its nature of high porosity despite very low permeability, a correction function was developed and employed. Then the resistivity profile was incorporated into estimation of fluid saturation in the reservoir by Archie's equation. The resultant values of porosity and water saturation are quite reasonable in comparison with reported ranges for this field.Assuming that the regional compaction being related to reduction of reservoir porosity has not been significant from 1997 to 2000 based on the ground subsidence in the area, we calculated a fluid saturation profile since the beginning of water flooding. We revealed the manners of fluid saturation change in the reservoir during water flooding through the time lapse analysis.

Integrating TDEM and MT methods for characterization and delineation of the Santa Catarina aquifer (Chalco Sub-Basin, Mexico)

Magnetotelluric (MT) and time domain electromagnetic (TDEM) surveys were undertaken in the region of Santa Catarina, located in the Chalco Sub-Basin of the Mexico Basin. The objective was to constrain the geometry of the fresh water aquifer and confirm the continuity of the basaltic flows between the volcano and the sedimentary basin. In order to define the stratification at depth with an emphasis on the geometry of the main aquifer, 11 MT and 5 TDEM soundings were recorded along a north–south profile. Interpretation of MT soundings show that the bedrock is located at a depth of at least 800–1000 m. Using TDEM apparent resistivity curves to constrain the high frequency MT data, three main layers were defined overlying the bedrock. These layers are, from the surface to bottom, a 20- to 40-m-thick layer of sands, ash and clay, followed by a very conductive 200-m-thick layer of sand and ash, saturated with highly mineralized water and, finally, a zone with gradually increasing resistivities, corresponding to the main aquifer. The TDEM data, the magnetic transfer functions and the 2D MT model also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This feature is likely to be highly permeable fractured basaltic flows, evidenced also in one of the water wells. To verify the presence of fractured basalts below the volcano ranges, 38 TDEM soundings were collected on the flanks of the Santa Catarina range. Layered models obtained from the TDEM soundings enabled an assessment of a major conductive zone (1–10 Ω m) at depth. Two hypothesis are envisaged and the nature of this zone is attributed either to a clayey layer or to fractured basaltic flows. If the latter possibility is confirmed, this continuous zone could provide a channel by which the water contaminated by the Santa Catarina landfill may leak into the basin.

Whole-space modeling of a layered earth in time-domain electromagnetic measurements

In-mine time-domain electromagnetic survey responses can be caused by conductive zones located either in the sequence above the mine drift, in the sequence below or in both. A computer code is developed that computes the whole-space transient electromagnetic field for a horizontal square loop in a layered earth. Four models are considered, which include conductive layer above, below or above and below the transmitter–receiver level. Both in-mine and transmission (transmitter on the surface and receiver at depth) configurations are modeled. The behavior of the vertical magnetic field or its time derivative is identical when the conductive layer is located either above or below the measurement station. On the contrary, the horizontal magnetic field or its time derivative display shape and polarity strongly dependent on the vertical position of the conductive layer, as well as on its conductivity, thickness and distance. Analysis of in-mine field data from the Taquari–Vassouras potash mine (Brazil) shows that no significant conductive layers are present below the mine gallery and that an usual interpretation technique may be used. A transmission test carried out with a transmitter loop at the surface and the receiver coil in the mine 400 m below demonstrates that the geoelectrical distribution above the mine, obtained from in-mine TDEM and borehole logs, is sufficient to satisfy the observed data.

Airborne TEM surveying with a SQUID magnetometer sensor

Traditionally airborne time-domain electromagnetic (AEM) survey systems use induction coils as the sensor (receiver). We have replaced the induction coil in a transient electromagnetic (TEM) system with a liquid-nitrogen cooled superconducting quantum interference device (SQUID) magnetometer sensor. Using this prototype system, we aimed to improve performance in detecting conductive mineralization, particularly where the conductive mineralization of interest is covered by a conductive regolith. We successfully demonstrated one- and three-component SQUID sensors in airborne TEM surveying, and achieved performance comparable to the induction-coil systems. Implementation of the SQUID system required development of devices capable of operating in magnetically unshielded environments with low noise, high slew rate, and wide bandwidth. Operation of the SQUID sensor in the highly dynamic environment of a towed bird was also necessary, and this implies a high dynamic range and high level of noise associated with the motion in Earth's magnetic field. The high dynamic range of the SQUID response was handled by a combination of resetting the SQUID flux locked loop, reducing the bandwidth, and providing high-gain feedback in parallel with the flux locked loop. A digital stacking filter was used to eliminate low-frequency noise associated with sensor motion. Isolation of the sensor from motion at the TEM signal frequencies required development of a sophisticated suspension system. The SQUID systems were tested over two known conductive targets, and their performance compared with the induction-coil TEM system. A comparative performance measure is developed to take the different sensitivities of the SQUID magnetometer and induction-coil receivers into account. This measure indicates that the SQUID system has superior performance for responses over earth structures with decay time constants greater than ∼6 ms when compared with the induction-coil signals. We also estimate the performance in comparison with integrated outputs of the induction-coil system and show that, at the demonstrated levels of SQUID performance, it is expected to have poorer performance by a factor of two or more. This disadvantage will be reduced for lower frequency, wider channel width TEM configurations or by improvements in the SQUID devices.

Geophysical Characteristics of Salinisation at Cape Portland, NE Tasmania

An integrated geophysical study of salinisation at Cape Portland, NE Tasmania, has mapped the distribution of saline areas, identified constrictions in the hydrogeologic basement and a possible source, store and transport mechanism for the salt. EM-31 data not only clearly delineates the extent of salt scalds, but also highlight areas of elevated conductivity not visibly affected by salt. EM-31 data maps the distribution of the salt at a much higher resolution than was previously possible using shallow drilling. Results from time-domain electromagnetic surveys confirm the responses seen in the EM-31 data and provide additional information about the subsurface distribution of conductive material. Conductivity depth pseudosections and layered earth inversions indicate depressions of up to 100 m in the resistive basement that are infilled with more conductive material. Ground magnetic data show the distribution of shallow Jurassic dolerite basement features and define a major negative anomaly that transects the study area. This feature is coincident with depressions identified from the time-domain electromagnetics and with a negative Bouguer gravity anomaly. The electromagnetic and potential field interpretations are consistent and indicate the presence of a major palaeochannel infilled with conductive material that cross-cuts the study area. Seismic refraction surveys and shallow auger holes in this zone provide additional support for the presence of a palaeochannel. This feature is inferred to be a major control on salinisation at Cape Portland.

The Geophysical Characteristics of the Trilogy Massive Sulphide Deposit, Ravensthorpe, Western Australia

The Trilogy deposit, located near Ravensthorpe in southern Western Australia, is a polymetallic massive sulphide deposit hosted within the conductive graphitic phyllites of the Proterozoic Mount Barren metasediments. There are two different styles of mineralisation, Pb-Zn massive-style and Cu-Au stringer-style mineralisation, contained within a silicified envelope that hosts the mineralisation.Petrophysical samples were taken and geophysical surveys conducted to characterise the response of the massive sulphide system and to help define an exploration model.Gravity, ultra-detailed airborne magnetic and radiometric, airborne time domain electromagnetic (EM) and ground time domain EM surveys have all been conducted over the deposit. The gravity survey outlined the position of the massive Pb-Zn mineralisation. A radiometric low around the surface projection of the silicified zone was evident in the airborne radiometric data. There was no response from the deposit recorded in the airborne magnetic survey. There is a subtle airborne EM anomaly over the known mineralisation, but the ground EM better defines its location and EM response. The airborne EM anomaly produced by the deposit was small compared with the response of the conductive phyllites elsewhere in the region.Geophysical exploration may aid in the search for additional sulphide mineralisation in the area. The combination of airborne EM with followup detailed gravity and ground EM has proven to be the most effective geophysical strategy in locating Trilogy style mineralisation in the Mount Barren basin.

Geophysical Characteristics of Salinisation at Cape Portland, NE Tasmania

An integrated geophysical study of salinisation at Cape Portland, NE Tasmania, has mapped the distribution of saline areas, identified constrictions in the hydrogeologic basement and a possible source, store and transport mechanism for the salt.EM-31 data not only clearly delineates the extent of salt scalds, but also highlight areas of elevated conductivity not visibly affected by salt. EM-31 data maps the distribution of the salt at a much higher resolution than was previously possible using shallow drilling.Results from time-domain electromagnetic surveys confirm the responses seen in the EM-31 data and provide additional information about the subsurface distribution of conductive material. Conductivity depth pseudosections and layered earth inversions indicate depressions of up to 100 m in the resistive basement that are infilled with more conductive material.Ground magnetic data show the distribution of shallow Jurassic dolerite basement features and define a major negative anomaly that transects the study area. This feature is coincident with depressions identified from the time-domain electromagnetics and with a negative Bouguer gravity anomaly.The electromagnetic and potential field interpretations are consistent and indicate the presence of a major palaeochannel infilled with conductive material that cross-cuts the study area. Seismic refraction surveys and shallow auger holes in this zone provide additional support for the presence of a palaeochannel. This feature is inferred to be a major control on salinisation at Cape Portland.

Factors controlling the configuration of the fresh-saline water interface in the Dead Sea coastal aquifers: synthesis of TDEM surveys and numerical groundwater modeling

TDEM (time domain electromagnetic) traverses in the Dead Sea (DS) coastal aquifer help to delineate the configuration of the interrelated fresh-water and brine bodies and the interface in between. A good linear correlation exists between the logarithm of TDEM resistivity and the chloride concentration of groundwater, mostly in the higher salinity range, close to that of the DS brine. In this range, salinity is the most important factor controlling resistivity. The configuration of the fresh–saline water interface is dictated by the hydraulic gradient, which is controlled by a number of hydrological factors.

Improvement in TDEM sounding interpretation in presence of induced polarization. A case study in resistive rocks of the Fogo volcano, Cape Verde Islands

A Time Domain Electromagnetic (TDEM) survey was carried out in and around the caldera of the Fogo volcano, Cape Verde Islands, to detect the low resistive structures that could be related to groundwater. A sign reversal in the sounding curves was encountered in central-loop measurements for the soundings located in the centre of the caldera along three main radial profiles. The negative transients are recorded in the early channels between 6.8 and 37 μs. Negative values in an early time transient is an unusual field observation, and consequently the first step was to check the data to ascertain their accuracy and quality. In the second step, three-dimensional (3D) effects are evaluated and ruled out in this zone, while an Induced Polarization (IP) phenomenon is observed using Direct Current (DC) sounding measurements. In the third step, the IP effect is called upon to explain the TDEM distortions; a Cole–Cole dispersive conductivity is found to be adequate to fit the field data. However, the more relevant one-dimensional (1D) model is recovered when both central-loop and offset-loop data are jointly taken into account, thus indicating that an effect of dispersive conductivity is necessary to explain the field data. The 1D electrical structure exhibits four layers, with decreasing resistivity with depth. Only the first layer is polarizable and its Cole–Cole parameters are m=0.85, c=0.8 and τ=0.02 ms for chargeability, frequency dependence and time constant, respectively. However, the Cole–Cole parameters deduced from TDEM forward modelling remain different from those deduced from DC/IP sounding. In this volcanic setting, this IP effect may be caused by the presence of small grains of magnetite and/or by the granularity of effusive products (lapillis). As a conclusion, it is shown that a modelling using different TDEM data sets is essential to recover the electrical structure of this area.

The Maggie Hays and Emily Ann nickel deposits, Western Australia: A geophysical case history

Geophysical techniques played a significant role in the discovery of the Emily Ann massive nickel-sulphide deposit and extensions to the Maggie Hays deposit, which are associated with komatiitic olivine cumulate ultramafic rocks, in an Archaean greenstone belt located about 500 km east of Perth, Western Australia.Detailed aeromagnetic surveys were used to outline komatiitic rocks and structures. Physical property measurements on drill core showed the mineralisation to be highly conductive and magnetic. Trial induced polarisation, audio magnetotelluric and time-domain electromagnetic (TEM) surveys indicated that the latter had the most potential for detecting the nickel-sulphide mineralisation.The Maggie Hays deposit comprises both disseminated and massive nickel-sulphides concentrated at the base of an ultramafic unit 200-500 m below the surface. A limited moving-loop TEM survey in 1992 located an anomaly immediately north of the main part of the deposit. Diamond drilling of this anomaly failed to discover extensions to the deposit or explain its source. In 1995, a fixed-loop TEM survey delineated an excellent response confirming the earlier moving-loop anomaly, which when drilled, resulted in the discovery of the Maggie Hays North zone 100 m below the surface.A moving-loop TEM survey resulted in the discovery of the blind, high-grade Emily Ann nickel-sulphide deposit 3 km north of the Maggie Hays deposit, at a depth of 120 m. Downhole TEM surveys aided delineation diamond drilling of the deposits with the location of extensions of mineralisation.High-powered, late-time moving-loop TEM, with fixed-loop TEM follow up, is currently being used routinely to explore for additional deposits. However, the highly conductive overburden response obscures the signal from bedrock conductors, which are often represented only as low-amplitude, late-time anomalies. Geophysical targeting is further complicated by the close proximity of highly conductive barren banded iron formation-hosted massive sulphides. Trial airborne EM surveys have detected Maggie Hays North, but not Emily Ann and probably not the Maggie Hays main zone.

Electromagnetic methods applied to exploration for deep nickel sulphides in the Leinster area, Western Australia

Nickel-sulphide deposits lying at the base of the regolith usually give strong, readily interpreted, surface time-domain electromagnetic (TEM) anomalies. In the Leinster area of Western Australia much of the prospective stratigraphy has been covered by TEM surveys and the strong anomalies have been drill tested. Deep nickel sulphides may be economic if they occur close to existing mining and processing infrastructure, but targeting the subtle TEM responses of these bodies requires skilful application and interpretation of TEM surveys. Conductive regolith can mask the response of deep nickel-sulphide orebodies to surface TEM systems. IP effects and other geological noise originating in the regolith can make recognition of weak bedrock anomalies difficult. Taking a slingram or out-of-loop reading can minimise regolith effects, and give a better response from sub-vertical bedrock conductors than in-loop readings. Downhole electromagnetic (DHEM) surveys in reconnaissance drill-holes increase the effective penetration depth of TEM, and can assist surface TEM interpretation in a prospect area. Despite weak surface TEM anomalism, DHEM surveys of deep drill-holes can yield precise information about the position and conductivity of nickel sulphide systems.

Geophysical responses over the Mount Ararate Prospect

Mount Ararat Prospect is hosted in Cambrian greenstones in western Victoria. Early exploration showed excellent correlation between the soil geochemical copper peaks and electromagnetic anomalies over a gossan. Follow-up drilling resulted in discovery of the Mount Ararat Prospect by Pennzoil in 1975. Massive banded and disseminated chalcopyrite-pyrrhotite-sphalerite mineralisation occurs in mafic and pelitic schists. The deposit is uneconomic, with an inferred resource of approximately 1 million tonnes with grades of 2.7% copper, 10 g/t silver and 0.6 g/t Chalcocite is common in a zone of supergene enrichment. Geophysical surveys, including airborne electromagnetic (INPUT) survey, vertical-loop electromagnetics, ground magnetic surveys, magnetic induced polarization, induced polarization and rapid reconnaissance magnetic induced polarization RRMIP, were used to delineate the extent of mineralisation. The INPUT survey and gradient-array induced polarization results were disappointing due to a very conductive surface layer. Magnetic induced polarization surveys were effective in delineating an anomalous conductive zone corresponding to the mineralisation. Downhole resistivity and induced polarization logs show the mineralised zone has coincident chargeability and resisitivity responses but the chargeability response is broader and more extensive indicating a possible disseminated pyrite halo in a broader zone around the massive sulphides. Experimental time domain electromagnetic surveys using the UTEM and SIROTEM systems recorded responses over the mineralisation but these techniques have not been routinely used for exploration. The early-time electromagnetic data is affected by a highly conductive near surface layer. The magnetic responses associated with the deposit are caused by the host rocks, and are not a direct response from the mineralisation. These units remain prospective along strike for base metals and gold.