TEM Surveys Research Articles

High resolution 3D subsurface mapping using a towed transient electromagnetic system ‐ tTEM: case studies

ABSTRACTGeophysical methods are routinely applied for investigation of near surface in areas of infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. A new towed transient electromagnetic tool, called ‘tTEM’, provides rapid, efficient, high‐resolution imaging of subsurface hydrogeology, and can deliver densely spaced profiles of resistivity. These profiles can be used to map a three‐dimensional subsurface in high resolution. In this paper, we present three case studies where the towed transient electromagnetic system was used to map the subsurface at the hectare scale. In the first case, we used towed transient electromagnetic to map raw materials in the northern Jutland, Denmark. The survey was carried out to map possible sand and gravel deposits. In the towed transient electromagnetic models, the potential sand/gravel targets are identified as high resistive bodies in the top 30 m. These bodies can have significant lateral variation at scales much less than one hundred metres. In the second case study, towed transient electromagnetic was used to map the thickness of a protecting clay layer above an aquifer in Vildbjerg, a town in the central part of Denmark. Results show that the overlying clay layer has sufficient thickness (>15 m) to protect the underlying aquifer from pesticide pollution in the area. Finally, in the third case study, we used towed transient electromagnetic for mapping geology in the vicinity of a landfill. The inversion results reveal a hitherto unknown buried valley‐like feature within the top 30 m of the subsurface that was not identified by older, regional TEM surveys – a feature that can have significant impact on the water flow.

Мониторинг фронта заводнения карбонатных коллекторов нижнего кембрия методом 4D электроразведки

Мониторинг фронта заводнения карбонатных коллекторов нижнего кембрия методом 4D электроразведки

CGG’S New Helitem-C AEM Systems

Recent development and re-design of the Helitem helicopter time domain system have resulted in the release of the Helitem30C and Helitem35C systems, both of which employ MultiPulse™. We describe the Helitem30C system, which was designed for efficient operation and is applicable to the majority of TEM surveys. The Helitem35C was designed to increase sensitivity to both resistive and conductive targets (increased geologic bandwidth) while achieving maximum power (and depth). A redesigned receiver suspension system significantly reduces coil motion at low frequencies, allowing for 12.5/15 Hz operation with no increase in coil motion noise, important for exploring under cover. We show case histories using both systems to illustrate their performance.

GEOPHYSICAL INVESTIGATIONS AT NAFPLION CITY, GREECE. HYDROGEOLOGICAL IMPLICATION

The application of selected geophysical methods within the framework of an Urban Geology investigation program performed by IGME at Nafplion city, contributed to the overall geo-scientific characterization of the study area. The methodology adopted included the application of Time Domain Electromagnetic, Electrical resistivity tomography, Gravity and Seismic Crosshole surveys. The results of the geophysical investigation provided quantitative information regarding the distribution of seawater intrusion in the area and determined factors controlling its extent within the investigated region. The intrusion may occur in three distinct horizons. The first within the shallow unconfined aquifer within the top 12 meters of fluvial deposits showing values of electrical conductivity in the range of 1 and 4 Siemens/m, corresponding to TDS values between 5000 and 20000 mg/L, as determined by the TEM surveys. Lower conductivity values between 0.3 and 1 Siemens/m are found within the second aquifer occurring between 15 and 45 meters. Similar order of conductivity values are found within the deeper horizon, lying below 45 meters depth, attributed also to seawater intrusion mainly in carbonate formations. Areas of fresh groundwater, were delineated by a combination of TEM, ERT and gravity methods. These areas are structurally controlled by faults and the presence of impermeable flysch overlying the limestone formation.

Current turn-off in an ungrounded horizontal loop: experiment and theory

An ungrounded horizontal loop, a common transmitter type in TEM surveys, makes up a system with distributed parameters with the earth under it. It can be simulated by an equivalent circuit with lumped parameters at late times and/or low frequencies, but at early times commensurate with the period of free current oscillations, the lumped circuit model fails to account for experimental data. At high frequencies and/or early times, the wire, in combination with the underlying earth, forms a transmission line in which current behaves according to the wave equation. This model allows calculating the current at any time and at any loop point with reference to the theory of long transmission lines. At early times, the loop self-responses depend on near-surface resistivity and environment and its primary magnetic field differs from that predicted by the classical theory of TEM surveys. Therefore, inversion of early-time rerponse in terms of the conventional TEM system model is meaningless. However, as illustrated with a loop shunted by a matching resistor, the loop model as a combination of two transmission lines enables the inversion of the early-time current response in terms of the line parameters and near-surface resistivity.

The Effects of Negative Values of Apparent Resistivity in TEM Surveys

The calculation of apparent resistivity in electrical methods is relatively simple and consolidated in the literature. Furthermore, in the case of DC methods, the apparent resistivity values are for the most part, intuitive and enable an initial interpretation of the results. On the other hand, in TEM method the apparent resistivity values are not very intuitive and interpretation based only on the apparent resistivity values cannot be done reasonably. In this way, this paper presents a discussion about the reasons and effects of the negatives values of apparent resistivity in TEM soundings. The main objective of this paper is to clarify the meanings of these negatives values and the possible effects in TEM inversion.

TEM surveys for search of taliks in areas of strong fast-decaying IP effects

Lenses of water-saturated unfrozen rocks (taliks) in permafrost are important sources of freshwater in high-latitude regions. Taliks stand out against the host frozen rocks in much lower resistivity and thus are detectable by resistivity surveys. TEM soundings are especially efficient in this application as they can go without galvanic grounding, have small offsets, and are sensitive to buried conductors. Early-time TEM data in the Taz area of the Yamal-Nenets district bear strong effects of fast-decaying inductively induced polarization (IIP), which rules out the use of nonpolarizable earth assumption for their interpretation. The TEM responses are inverted by means of the TEM-IP software using the model of a polarizable earth with Cole-Cole complex frequency-dependent conductivity. The resulting earth model mainly includes three layers, with a 100 to 250 m thick highly resistive polarizable upper layer. The polarization parameters of the layer (chargeability, time constant and exponent) are typical of frozen sedimentary rocks, while the presence of a talik reduces notably the effective resistivity and chargeability. This feature can be used as a guide to taliks, as it was confirmed by TEM surveys and subsequent drilling.© 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Testing TEM systems using a large horizontal loop conductor

Testing TEM systems has to include field experiments with physical models commensurate to the real transmitter–receiver configurations and to the target subsurface features. A large horizontal loop closed across a known resistance is a convenient model in this respect. It is convenient to lay in the field, it has manageable parameters, and its natural response is easy to calculate.A field-size experiment and numerical modeling were applied to investigate the model wire loop response to eddy current in a uniform conductive ground, both at early and late times. The higher the resistivity of the ground the larger the time range in which the measured response matches the predicted one, other things being equal.The experiments show that (i) closed loops laid near a transmitter–receiver system are applicable to test the quality of the latter as a tool for TEM or other similar resistivity surveys; (ii) current induced in the model loop can be used to infer the resistivity of the ground; (iii) a closed loop slows down the growth and reduces the amplitude of voltage induced at early times in a receiver loop or in a multiturn coil.

Magnetic relaxation of a horizontal layer: Effect on TEM data

We have modeled central-loop and coincident-loop transient responses of a magnetically viscous layer sandwiched between two nonmagnetic ones. The coincident-loop transients show exponential voltage decrease (at a fixed delay time), at any thickness h 2 of the magnetic layer, with an increasing depth to the latter ( h 1) or the loop height if the layer is exposed on the ground surface. The patterns of central-loop transients are different from those of the coincident-loop ones and from one another for thin and thick magnetic layers. Namely, the voltage first rises to its maximum and then falls as the depth to the magnetic layer ( h 1) increases, if it is thin: the thinner the layer, the more prominent the peak. If the layer is thick, the voltage decreases monotonically with its depth (or with loop height above the ground). Voltage grows, first rapidly and then progressively more slowly, at ever greater thicknesses of the magnetic layer in both loop configurations. At large h 2, the effect from the magnetic layer becomes similar to that from a magnetically viscous halfspace. These features of the transient responses have to be taken into account in planning and conducting TEM surveys, as well as in a geological interpretation of the TEM data affected by natural and/or man-caused magnetically viscous ground. In the general case, the turn-off of the transmitter current induces eddy current in the ground beneath the loop, which decays at a rate proportional to the ground resistivity. The eddy current decay and magnetic relaxation processes being independent at conductivities (resistivities) common to the real subsurface, the effect of the former can be allowed for using the superposition principle. This principle implies that the total response of a magnetically viscous conductor is a sum of the magnetic relaxation and eddy current components.

TEM surveys for magnetic viscosity of rocks in situ

We discuss the results of a field experiment in the Malaya Botuobiya area (West Yakutia) at a site where earlier surveys revealed slowly decaying transient responses. That time-dependent voltage decay indicated magnetic viscosity effects associated with magnetic relaxation of superparamagnetic grains in rocks. In this study, we have applied a high-resolution array TEM survey to contour the anomaly and parametric soundings with systems of different configurations to explore the vertical pattern of magnetic viscosity. The parametric data have been inverted, by means of manual and automated fitting, with a reference model of a layered magnetically viscous earth, using, respectively, analytical formulas and simulation based on a forward solution by separation of variables. According to both automated and manual inversion, the section at the center of the anomalous site fits a three-layer earth model with an intermediate magnetically viscous layer between two nonmagnetic layers. This model is consistent with a priori evidence of local geology and may provide more details of the latter. The inversion results have been further used to estimate the volumetric percentage of superparamagnetic grains in the magnetically viscous layer, assuming magnetite to be the main ferrimagnetic phase.

Significantly increasing TEM survey performance by modifying frequency content of transmitter waveform

TEM surveys typically operate with a simple transmitter waveform such as a 50% duty cycle alternating square wave. The frequency of transmission and the duration of measurement is decided before the survey or, frequently in the case of ground TEM surveys, adapted during the survey by the operator to suit the conditions.

Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana

One hundred meter square in-loop ground TEM soundings were an effective way to screen aeromagnetic, ground gravity, and Falcon gravity gradiometer signatures caused by kimberlite intrusions overlain by 40-120 meters of transported Kalahari sedimentary cover in the Kokong kimberlite field of Botswana. Ground TEM?s effectiveness in identifying kimberlite pipes led to the flying of the VTEM aeroTEM system over selected areas at Kokong. Ten kimberlites previously covered by ground TEM surveys were over flown by the VTEM survey. A comparison of the ground TEM and VTEM responses show that the VTEM effectively drill-screened nine out of the ten kimberlite magnetic or gravity signatures, whereas the ground TEM systems effectively screened all 10 kimberlite signatures. KS40, the largest kimberlite at Kokong and the one kimberlite less well mapped by the VTEM was, at 10-30 msec, a later-time conductor that best responded after the VTEM?s last channel 27 at 7.5 msec. KS40?s kimberlite crater was mapped as a good late-time conductor (later than 7.5 msec) by both ground TEM systems. The ground TEM systems employed were a Geonics TEM57 transmitter and PROTEM receiver, and two Zonge ZT-30 transmitters and GDP32 receivers.

Detection of Sea Water Ibtrusion Caused by Tidal Action Using Geophysical Methods

The 1 km2 studied area is located in Sukchun-ri, Hwasung-koon, the southern part of kyeonggi province of Korea. In order to determine the extent of seawater contamination and a preferred channel of the seawater intrusion, DC resistivity and TEM surveys were performed. According to the resistivity map obtained from geophysical surveys, the study area is divided into two districts as relatively low (less than 30 ohm-m) and high (more than 30 ohm) areas. The distribution of the low resistive area is consistent with the distribution of the layer of composed pf clay minerals, and the resistivity of the clay miner layer decreases slowly as approaching to the old seashore. Hydrogeological analysis shows that the clay layer within a distance of about 200 m from the seashore has been already contaminated by sea water, and its electric conductivity is 8 times higher than that of the sand layer covered by the clay layer. According to the results of the 2-dimentional DC resistivity surveys with a dipole-dipole array, there are two preferred channels of seawater intrusions in the site, and both channels are in NW-SE direction from the old seashore. The DC resistivity and TEM monitorings were carried out along the preferred channel which has low resistivity zone extended to a depth of 80 m. The time series measured by those two methods fluctuates with a period of 12 hours. These observations show that the sea water intrusion caused by tidal action is still in progress along the preferred channel interpreted by the geophysical surveys.

Mapping fly‐ash water pond leakage with TEM and IP data at Loy Yang coal‐mine (Australia)

The fly‐ash water pond at the Loy Yang power station (Latrobe Valley, Victoria, Australia) seeps and therefore presents a hazard to the local environment from its high total dissolved solids content. A transient electromagnetic survey (TEM) was completed to investigate the contaminant plume in more detail.The contaminate plume was successfully delineated; however, multiple contiguous soundings featured late‐time negative decays not typically recorded in coincident‐loop TEM surveys. This indicates that conductivity is frequency‐dependent. These anomalous soundings suggested the presence of a shallower contaminate plume than that mapped by the boreholes and therefore further analysis was required. An induced polarization survey revealed that the area where the late‐time negative responses were recorded coincided with a shallow conductive and extremely chargeable anomaly. A simple power‐law relationship holds between the negative transients decay rates and normalized apparent chargeability. Analysis on soil samples obtained from auger holes related the polarization anomaly to clay‐, organic‐ and metal‐rich layers of fill material.

The deep geothermal structure of the Mid-Atlantic Ridge deduced from MT data in SW Iceland

Iceland is very active tectonically as it is crossed by the Mid-Atlantic Ridge and its associated rift zones and transform faults. The high-temperature geothermal systems are located within the neo-volcanic zone. A detailed comparison of the main features of the resistivity models and well data in exploited geothermal fields has shown that the resistivity structure of Iceland is mainly controlled by alteration mineralogy. In areas where the geothermal circulation and related alteration take place at depths of more than 1.5 km, the investigation depth of the DC and TEM methods is inadequate and the MT method appears to be the most suitable survey method. MT soundings were carried out to determine the deep structure between two neighboring Quaternary geothermal fields: the Hengill volcanic complex and the Brennisteinsfjoll geothermal system, both known as high-temperature systems. MT data were analyzed and modeled using 1D and 2D inversion schemes. Our model of electrical conductivity can be related to secondary mineralization from geothermal fluids. At shallow depths, the resistivity model obtained from the MT data is consistent with the general geoelectrical models of high-temperature geothermal systems in Iceland, as revealed by shallow DC and TEM surveys. The current MT results reveal the presence of an outcropping resistive layer, identified as the typical unaltered porous basalt of the upper crust. This layer is underlain by a highly conductive cap resolved as the smectite–zeolite zone. Below this cap a less conductive zone is identified as the epidote–chlorite zone. A highly conductive material has been recognized in the middle of the profile, at about 5 km depth, and has been interpreted as cooling partial melt representing the main heat source of the geothermal system. This conductor may be connected to the shallow structure through a vertical fault zone located close to the southern edge of the profile.

The Waterloo and Amorac nickel deposits, Western Australia: A geophysical case history

The Waterloo and Amorac nickel sulphide deposits are significant discoveries located in the north eastern goldfields region of Western Australia. They were found as a result of the systematic application of geophysical techniques during a green fields exploration program. The deposits are 300m from each other, blind, and are overlain by 80m of deeply weathered basement rocks. Despite their close proximity, the deposits are quite separate and different in style. Interpretation of detailed aeromagnetic data was first used to delineate komatiite-basalt contacts of interest for nickel sulphide exploration. Then these contacts were surveyed with a moving loop TEM survey (Inloop and Slingram receivers) which delineated nine basement conductors. Finally, fixed loop TEM surveys were used to better define the targets for drill testing. Drill hole TEM surveys have been used since the discovery to guide drilling and better define the mineralisation size, location, orientation, and character. Complicated geometry and multiple conductive sources make careful interpretation and modelling essential.

The geophysical signature of the Paleoproterozoic Menninnie Dam lead-zinc-silver prospect, South Australia

The Menninnie Dam lead-zinc-silver prospect was discovered through follow-up of anomalies defined by an airborne magnetic survey flown as part of a programme to explore for Broken Hill type deposits in the Palaeoproterozoic rocks of the northern Eyre Peninsula, South Australia. Shallow reconnaissance drilling indicated the presence of base-metal mineralisation, and subsequent surface geophysical surveys, including moving- and fixed-loop TEM, gravity and ground magnetics, confirmed the presence of large magnetic and conductive sources. After drill testing, the magnetic sources were found to be banded iron formations, whilst the conductors were caused by a combination of clays and saline water, overlying deeply weathered massive sulphide zones. The regolith created major difficulties in data interpretation due to its highly conductive nature and the deep weathered profile across the mineralisation. Surface moving- and fixed-loop TEM surveys were used in an attempt to locate the source(s) of the strong conductivity anomaly, both in plan and in section. Neither method was clearly able to differentiate between the deeply weathered regolith and the underlying sulphide mineralisation.

Some Positive thoughts about Negative TEM Responses

Negative responses recorded during moving in-loop TEM surveys, where only positive data are expected, are a regular occurrence in TEM surveys. They are however generally poorly understood and little reported. In most cases, such negatives are ascribed to induced polarisation (IP) effects, but we suggest that there is at least one other cause of negative TEM response, one which is associated with very resistive terranes containing little if any polarisable material. This paper presents some examples of this negative resistive response where there is little or no polarisable material present and where a source other than IP should be sought. A series of tests have been proposed to try and isolate the cause.

Tritton Copper Deposit, Girilambone NSW. A Geophysical Discovery

The Tritton massive sulphide copper deposit was discovered using systematic moving loop surface TEM surveys. The use of this technique was based on previous experience around the Girilambone copper mine. The deposit consists of an upper and lower lens, both of which are highly conducting. The lower lens was discovered through the use of downhole EM techniques.The upper lens lies at a depth in excess of 160 m below the surface and the lower lens is at a depth of greater than 400 m. Both these lenses have EM time constants of about 10 ms.Apart from having a minor associated magnetic anomaly the Tritton deposit has not been detected from the surface by any other geological, geochemical or geophysical technique.Approximately one kilometre north of Tritton is the uneconomic sulphide deposit, Budgerygar. This has similar geometry to the Tritton deposit but lies at the shallower depth of 60 m. The Budgerygar deposit is readily detectable by a variety of geochemical and geophysical techniques.The Tritton and Budgerygar deposits are an interesting test site for the development of new exploration technology, due to the similarity in their form but difference in depth and hence detectability. A number of geophysical techniques have been tested over the Tritton deposit since its discovery.

Short Note: The Location of TEM Transmitter Loops Underground and in Rugged Terrain

The size, shape and location of transmitter loops is of fundamental importance in the design of both surface and drillhole TEM surveys. The shape and location of the loop, with respect to a target conductor, strongly affects the level of electro-magnetic coupling between the loop and the conductor. The effect of sloping terrain on the coupling field of a surface loop is shown for several terrains. In the case of transmitter loops located underground, both maximum and minimum coupling can be obtained at several positions and attitudes of conductors located around the loop. Transmitter loops located on sloping terrain or in inclined mine workings can often provide greater control over the level of energisation of the target conductor than loops located on a flat terrain. In these circumstances it is possible to determine conductor location and attitude with greater accuracy. Computing the electro-magnetic coupling fields of transmitter loops, for various attitudes of the target conductor, is mandatory when designing TEM surveys and interpreting survey data.