Time Domain Electromagnetic Research Articles

A hydrogeophysical study to estimate water seepage from northwestern Lake Nasser, Egypt

Estimating the water seepage from Lake Nasser, southern Egypt, into the adjacent Nubian sandstone aquifer is one of the main factors influencing the water balance of the lake. Up till now, there has been no information about seepage values, particularly in the northwestern part of the lake, due to a complete absence of boreholes. The present study is an approach using the time domain electromagnetic (TDEM) technique to estimate the hydraulic parameters of the shallow Nubian sandstone aquifer which are essential in water seepage calculations. The average porosity of the Nubian sandstone aquifer is calculated using bulk resistivities deduced from a TDEM model by applying the Archie formula. As calculated porosity values are consistent with laboratory measurements, accordingly the void ratio is estimated from the calculated porosity. The calculated void ratio in combination with the grain size diameter of the shallow part of the Nubian sandstone is used to estimate hydraulic conductivity. Then Darcy’s low is applied to calculate the seepage value of the lake water in the adjacent Nubian sandstone aquifer. The main result of the current study shows that the anticipated water seepage value is 2.6 × 10 6 m 3 yr –1 . This value seems reasonable in comparison with the total seepage value inferred from isotope studies around Lake Nasser.

Computational Modeling of Uncertainty in Time-Domain Electromagnetics

We discuss computationally efficient ways of accounting for the impact of uncertainty, e.g., lack of detailed knowledge about sources, materials, shapes, etc., in computational time-domain electromagnetics. In contrast to classic statistical Monte Carlo--based methods, we explore a probabilistic approach based on high-order accurate expansions of general stochastic processes. We show this to be highly efficient and accurate on both one- and two-dimensional examples, enabling the computation of global sensitivities of measures of interest, e.g., radar-cross-sections (RCS) in scattering applications, for a variety of types of uncertainties.

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.

Sub-Audio Magnetic Surveying for Shallow Occurrences of Conductive Manganese Ore, Woodie Woodie Area, Western Australia

Manganese (Mn) ore in the Woodie Woodie area occurs as podiform ore bodies in Archaean dolomite and chert breccia thought to be 2.5 billion years old. Electromagnetic conductivity mapping using time-domain helicopter electromagnetics (HoistEM) surveying successfully detected six new discoveries under regolith and host rock cover in the area. Sub-Audio Magnetic (SAM) surveys were carried out over known EM-responsive Mn deposits to test the ability of this technology to detect conductive, highgrade manganese deposits as equivalent magnetometric resistivity (EQMMR) anomalies. SAM survey data were compared to other geophysical methods over five EM-responsive manganese deposits, which varied in size and burial depth. The methods compared were high-resolution gravity, HoistEM, gradient-array induced polarization (GAIP), dipole-dipole induced polarization (DDIP), and ground time domain electromagnetic (TEM) surveying. The SAM technique demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at shallow depth, usually less than 40 m. Podiform ore zones below 40 m depth did not produce EQMMR anomalies. The SAM depth of penetration is limited by the ability for current to be channelled into conductive, podiform ore zones sitting in resistive host rocks, below a moderately conductive regolith cover.

Groundwater development in Lemoa, Guatemala

In the rural Guatemalan Central Highlands, drinking water wells are a rare commodity due to lack of data regarding depth and sustainability of groundwater. At a study well in Lemoa, aquifer sustainability and other characteristics were estimated using hydrograph data and Theis’s recovery method for pumping tests. These tests showed that while specific capacity remained fairly constant, transmissivity increased, indicating development of the well subsequent to installation of the study well. The time domain electromagnetic (TDEM) method was evaluated at the site as a prospective groundwater prospecting technique. Characterization of subsurface conditions using TDEM is feasible with local subsurface data for correlation; however, it is limited to delineating low electrical resistive zones without geologic data for correlation. Data showed that groundwater produced from volcanic alluvium at approximately 79 m in depth is sustainable at the study area at current use rates.

Hydrothermal system beneath Aso volcano as inferred from self-potential mapping and resistivity structure

We conducted self-potential (SP) surveys sequentially from part to part over the central cones of Aso volcano since August 1998 by December 2001. The compiled SP map revealed large SP anomalies on the central cones. The main feature of the SP map is a ‘W-shaped’ profile along the NS-transect over the central cones. It is probable that this characteristic SP profile is produced by the combination of hydrothermal upwelling in the middle and topographic effect. A positive anomaly showing a large concentric pattern has appeared after correcting the topographic effect. To evaluate this SP anomaly, we implemented a numerical code that calculates electric potential produced by arbitrarily positioned current sources and sinks in any three-dimensional resistivity structure. A layered structure obtained from a time-domain electromagnetic (TDEM) field experiment was used for the resistivity model. The estimated current source is 300 A, being located in a conductive layer around the sea level. Meanwhile, sinks were estimated to sit on a circular area corresponding to the marginal part of the conductive layer. Water and heat budget study gives a lower limit of water mass transfer from depth to the bottom of the crater lake of Nakadake. This value was used to estimate the equivalent current in either case of electro-kinetic (EK) [Mizutani, H., Ishido, T., 1976. A new interpretation of magnetic field variation associated with the Matsushiro earthquakes, J. Geomag. Geoelectr., 28, 179–188.] or rapid fluid disruption (RFD) process [Johnston, M.J.S., Byerlee, J.D., Lockner, D., 2001. Rapid fluid disruption: A source for self-potential anomalies on volcanoes, J. Geophys. Res. 106(B3), 4327-4335.]. This comparison suggests that the former process is preferable to explain the observed SP anomaly. From these results we infer a large-scale hydrothermal system beneath the central cones of Aso volcano, in which the fluid flow initiates from the surrounding area, converging to the central vent to transport the heat and materials up to the crater lake of Nakadake through a vapor-filled conduit.

The use of the time domain electromagnetic method to delineate saline groundwater in granular and carbonate aquifers and to evaluate their porosity

The use of surface geoelectric (GE)/geoelectromagnetic (GEM) methods to determine the porosity of freshwater saturated aquifers using Archie's equation leads in many cases to significant inaccuracy. This is caused by the uncertainties in determining both fluid and bulk resistivities required for calculating porosity of a rock sequence in the case of a known rock type. In order to solve the problem in coastal and other saline-water-intruded aquifers, it is proposed herein to apply the time domain electromagnetic (TDEM) method to detect first the fresh/saline water interface and then to determine the fluid and bulk resistivities of the saline water saturated portion of the aquifer. It is shown that calculating porosity of this portion of the aquifer is much more accurate than that of the freshwater saturated part. Since the porosity of the aquifer is generally invariant with regard to the location of the interface, the proposed method in many cases is applicable to the entire aquifer. The method was tested in both granular and carbonate coastal aquifers of Israel saturated with normal seawater as well as with Dead Sea concentrated brines. The cases of granular coastal aquifers exhibit a good agreement between measured porosities and those calculated using TDEM results. In the case of carbonate aquifers, the situation is more complicated due to their heterogeneity and much wider range of porosities stemming from their karstic and fractured nature.

Electrical and EM methods, 1980–2005

The last two decades saw major advances in data collection, processing, and interpretation of electrical and EM data. Lower transmitter frequencies for airborne time-domain EM systems have made possible surveys in areas where conductive cover previously screened basement conductors. As with every other branch of technology, the evolving speed of the silicon chip and of streaming data to hard disk has revolutionized data collection and noise reduction processing. Major advances have been made on increasing the signal-to-noise ratios in ground EM data acquisition systems. Full-waveform recording and the use of multiple receivers are becoming common for ground EM techniques. Previously intractable 2D and 3D data inversions are now slowly becoming available. Finally, controlled-source EM techniques are now being used to detect and characterize hydrocarbon-bearing reservoirs in deepwater areas.

The hydraulic and electrical fractal dimension of regolith

Electrical and electromagnetic (EM) geophysical data from a line or grid are usually converted into two- or three-dimensional resistivity models of the Earth. In regions of relatively uniform geology, variability of measurements are assumed to be random and Gaussian, such that the Earth models are found from a maximum likelihood approach. In this paper, we show that such variability is not Gaussian, and has a scale-length dependence, expressed as a fractal dimension. We argue that the fractal dimension of electrical and EM data is causally determined from Earth heterogeneity, which provides a link with hydraulic conductivity in porous and fractured media that also has a fractal dimension. This paper presents initial analyses of data from two sites within South Australia. NanoTEM time domain electromagnetic (TEM) data were collected at Tunkillia in the Gawler Craton with a target of identifying palaeochannels for gold exploration. Analysis of survey data revealed a good correlation between regions of low resistivity and high fractal dimension. On the other hand, a river-borne NanoTEM survey of the River Murray sediments showed much lower correlation. We conclude that analysis of the variability of EM data may provide useful additional constraints on subsurface properties.

Discoveries through innovation in applications of airborne and ground TDEM in very conductive environment

In the Buffalo Head Hills diamond province of Alberta, Canada, kimberlites have intruded a thick sequence of sedimentary rock units with thickness in excess of 300-500 m and resistivities of 5?10 ohm-m. In contrast to the conductive sedimentary rock the kimberlite intrusion is invariably characterised by a much higher electrical resistivities. These ``resistive' kimberlites can be detected by both airborne and ground time domain EM methods. Because of the host sedimentary rock?s very low resistivity, the resistive kimberlite response in many cases appears counterintuitive to the expectations based on the simpler analysis of the problem which ignores the EM interactions between a 3D body and a conductive host. For similar reasons, successful detection of kimberlites in the Buffalo Head Hills province also required developments of algorithms which facilitate correction of airborne TDEM data for variations in aircraft altitude and pitch. ``Anomaly hunting' analysis which decompose the spatial and temporal characteristics of the EM response into a number of components and innovations in the inversion and transformations of ground TDEM data sets was also required. Application of standard Conductivity-Depth-Image (CDI) techniques was not appropriate to the solution to the problem. A new pseudo 1D-inversion algorithm was developed to partially assist in the assessment of the data. A number of areas previously covered by detailed airborne magnetic surveys were re-surveyed with the Geotem airborne TDEM system. Use of the developed concepts and insights has resulted in a number of new diamonds-bearing non-magnetic kimberlite discoveries. New discoveries, for example, include kimberlites K296 (with a surface area of some 500 by 500 m), and K252 (with an estimated minibulk sample diamond content of 55.0 cpht), the highest estimated diamond content of all Alberta kimberlites to date.

The emerging role of helicopter time domain EM systems

Helicopter time domain electromagnetic (H-TEM) systems are an emerging technology. The ability to integrate the great depth penetration of fixed-wing systems with the spatial resolution and conductance discrimination of helicopter frequency domain systems is an important development for the detection of discrete conductors. H-TEM systems can be configured with multiple receiver coils to make their responses more diagnostic, such as the discrimination of thick from thin conductors. The use of a rigid geometry allows these systems to measure during the transmitter on-time where high conductance targets produce a stronger response. In areas of conductive overburden, it is possible to remove much of the high amplitude response due to surface conductivity from deeper bedrock conductors by subtracting the off-time response from the corresponding on-time response. The resultant profiles are more diagnostic of bedrock conductors than are the late off-time channels from the same system at a lower base frequency.

Towards seawater depth determination using the helicopter HoistEM system

Interpretation of helicopter time-domain EM data from a 2002 HoistEM survey of Sydney Harbour is under way to define seawater depth. Forward modelling, conductivity-depth imaging (CDI), and ID inversion have been applied. The effectiveness of airborne EM bathymetry can be appraised by comparing interpreted seawater depths with accurate bathymetric soundings.An extensive comparison of modelled and observed HoistEM responses over deep (~33 m) and shallow (~6 m) seawater showed that good agreement could be achieved for models with the correct depth of water if the seawater conductivity was assigned an unrealistically low value of about 3.5 S/m. The actual seawater conductivity in Sydney Harbour is 4.65 + 0.15 S/m.CDI processing reinforced the conclusion that the measured HoistEM responses are incompatible with the known seawater conductivity. However, a qualitative agreement was observed between the CDI seafloor topography and the true bathymetry. In an effort to reconcile the data with the “ground truth”, transmitter moment and altitude were adjusted, for a given seawater conductivity, prior to CDI processing.ID inversion of HoistEM data yielded sub-metre water-depth accuracy for depths shallower than 22 m, if an unrealistically low seawater conductivity was allowed. Thus forward modelling, CDI, and initial ID inversion results all suggest that the HoistEM system is viable for bathymetric surveys but that rescaling of the Sydney Harbour dataset will be required in order to reconcile the measured responses with the known seawater conductivity.

Electromagnetic detection of deep freshwater lenses in a hyper-arid limestone terrain

In the hyper-arid desert of Central Oman, freshwater lenses are found lying on a regional saline water table. These lenses have developed where recharge from infrequent cyclonic rainfall has collected in shallow depressions on the Tertiary limestones of the Central Plateau and in the catchments of ancient river channels draining the Plateau. Central-loop time-domain electromagnetic (TDEM) sounding was applied as a method of reconnaissance exploration for these lenses at two sites, a shallow depression extending over an area of 60 km 2 and a wadi gorge draining a catchment of 3400 km 2. These results were subsequently tested by drilling. In the case of the shallow depression, drilling intersected a freshwater lens up to 18 m thick at a depth of 92 m. TDEM resistivity-depth inversion showed that the corresponding high resistivity zone included both the lens and overlying unsaturated rocks, and that the depth to the saline interface could be accurately predicted. Where drilling failed to intersect a lens, TDEM inversion resulted in a consistently low resistivity zone in which the water table could not be resolved. By invoking the Archie formula modified for the presence of clays, it is thought that the higher resistivity of the vadose zone observed over the lens may be explained by a reduction in the clay conductivity factor resulting from higher pore-water resistivity. In the case of the wadi gorge, low regional resistivities were also recorded over the limestones on the survey margins, and high resistivity anomalies over the freshwater lens within and extending away from the gorge. Again, TDEM was found to be useful as a reconnaissance method and for mapping the depth to the underlying saline aquifer, but not for predicting the thickness of the overlying freshwater lens.

Recent System Applications of Short-Pulse Ultra-Wideband (UWB) Technology

Developed in the early 1960s, time-domain electromagnetics, the study of electromagnetic-wave propagation from a time-domain perspective, has given birth to a fascinating new technology, which today is commonly referred to as ultra-wideband (UWB). It has now been slightly more than 25 years since the 1978 seminal paper of Bennett and Ross, which summarized UWB's early applications. It thus seems appropriate, given the tremendous increase in interest in the technology since the Federal Communications Commission modified its Part 15 rules to accommodate UWB transmissions, to take a look at more recent system applications of this unique technology. This paper provides a brief historical perspective of UWB, discusses recent techniques for the generation and reception of short-pulse electromagnetic waveforms, and examines a number of recently developed UWB systems in the communications, radar, and precision-positioning fields. Finally, a brief assessment of future trends for the technology is provided.

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.

Discussion on “Peculiarities of SQUID magnetometer application in TEM” (G. Panaitov, M. Bick, Y. Zhang and H.‐J Krause, GEOPHYSICS, 67, 739‐745)

Panaitov et al. (2001) describe certain peculiarities in measurements of time-domain electromagnetics (TEM) using superconducting quantum interference device (SQUID) sensors. In particular, they propose an explanation for sign reversals observed with the in-loop configuration in terms of remnant responses from earlier transmitted pulses of the repetitive transmitter waveform, and show several examples to demonstrate this effect. However, it appears that the data presented in their article are highly selective, and if realistic waveforms are used, sign reversals cannot be caused by the mechanism proposed.

Archaeological geophysics: 3D imaging of the Muweilah archaeological site, United Arab Emirates

The sand-covered Muweilah archaeological site in the United Arab Emirates (UAE) is a unique Iron Age site, and has been subject to intensive investigations. However, excavations are time consuming and may require twenty years to complete. Thus geophysical surveys were undertaken with the objective of characterising the site more expeditiously. This paper presents preliminary results of these surveys.Ground penetrating radar (GPR) was tested as a primary imaging tool, with an ancillary shallow time domain EM (MetalMapper) system. Dense 3D GPR datasets were migrated to produce horizontal (plan view) depth slices at 10 cm intervals, which is conceptually similar to the archaeologists’ excavation methodology. The objective was to map all features associated with anthropogenic activity. This required delineating extensive linear and planar features, which could represent infrastructure. The correlation between these and isolated point reflectors, which could indicate anthropogenic activity, was then assessed. Finally, MetalMapper images were used to discriminate between metallic and non-metallic scatterers.The moderately resistive sand cover allowed GPR depth penetration of up to 5 m with a 500 MHz system. GPR successfully mapped floor levels, walls, and isolated anthropogenic activity, but crumbling walls were difficult to track in some cases. From this study, two possible courtyard areas were recognised. The MetalMapper was less successful because of its limited depth penetration of 50 cm. Despite this, the system was still useful in detecting modern-day ferruginous waste and bronze artefacts.The results (subject to ongoing ground-truthing) indicated that GPR was optimal for sites like Muweilah, which are buried under a few metres of sand. The 3D survey methodology proved essential to achieve line-to-line correlation for tracking walls. In performing the surveys, a significant improvement in data quality ensued when survey areas were flattened and de-vegetated. Although MetalMapper surveys were not as useful, they certainly indicated the value of including other geophysical data to constrain interpretation of complex GPR features.

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.

Mapping saline groundwater beneath the Sea of Galilee and its vicinity using time domain electromagnetic (TDEM) geophysical technique

Goldman, M., Gvirtzman, H., and Hurwitz, S. 2004. Mapping saline groundwater beneath the Sea of Galilee and its vicinity using time domain electromagnetic (TDEM) geophysical technique. Isr. J. Earth Sci. 53: 187‐197.

The Moonta-Wallaroo mining district, South Australia: A geophysical perspective

Located on the Yorke Peninsula in South Australia, the Moonta-Wallaroo area is an important area of historical copper mining. Hydrothermal copper-gold-silver sulphide mineralisation occurs in Palaeoproterozoic and Mesoproterozoic metamorphic and igneous rocks, but there is no outcrop of these basement rocks due to subdued relief and extensive sedimentary cover. The lack of outcrop, plus the complexity of the local geology, means that basement geological maps of the area are largely speculative. An interpretation of semi-regional aeromagnetic data from the area suggests the basement comprises meta-igneous rocks overlain to the east by an increasing thickness of sedimentary and/or meta-sedimentary cover. Anomalies interpreted to be of stratigraphic origin are quite well developed in areas of meta-sedimentary rocks, and regional strike is interpreted as trending northeast-southwest. Linear anomalies to the north and west of the Wallaroo Mines are interpreted as resulting from magnetite deposited in faults during a regional iron-rich metasomatic event. Based on this interpretation, a major east-west/northeast-southwest trending deformation zone is recognised, interpreted as a locus of reverse and dextral faulting. The entire study area is cross-cut by late brittle faults that trend mainly northwest-southeast. This interpretation/structural scenario is consistent with mine-scale structural studies in both the Moonta and Wallaroo Mines, although, the resolution of the aeromagnetic data is insufficient to map deposit-scale structures. Ground-based geophysical exploration in the Moonta-Wallaroo area is hindered by the generally electrically conductive environment and the presence of cultural features, such as wire fences. However, surface and downhole time-domain EM surveys have successfully located massive-sulphide mineralisation comprising the Poona and Wheal Hughes deposits. The Poona deposit has also been shown to give rise to an IP response, and there is evidence that the lodes elsewhere in the Moonta area are also associated with IP anomalies.