Transient Electromagnetic Soundings Research Articles

Evaluation of the transient electromagnetic geophysical method for stratigraphic mapping and hydrogeological delineation in Campos Basin,Brazil

The applicability of the Transient Electromagnetic (TEM) geophysical method for stratigraphic mapping and hydrogeological delineation has been tested in terrestrial part of the Campos Basin in Brazil. The study area is located within the fluvial plain of Paraiba do Sul River, at the W edge of the continental portion of the Basin, geologically consisting of Tertiary and Quaternary sediments and a gneissic basement. Around 32 central loop TEM soundings were made with 10 or20 m-sided transmitter loops, to map the conductive and/or resistive zones in the subsurface, which were correlated with lithology from a drilled borehole in the test area, whose location was indicated by the present study. In general, the one-dimensional (1D) models constructed for the TEM field data suggest a 3-layer resistivity structure characterized by a resistor-conductor-resistor pattern. The shallow section of 100-500 ohm.m and 50 m average thickness is interpreted as correlating with clay and shaly sand formations. The intermediate layer, with thickness around 20 m and resistivity less than 10 ohm.m, corresponds to clay. The basal resistor of 10-100 ohm.m at depths of more than 60 m at most sites is interpreted as the combined signature of the basal conglomerates, shaly sands and crystalline basement. Comparison with borehole data shows that the zone corresponding to the conductive geoelectrical units contains many thin layers of clay and sand that cannot be individually resolved by the TEM data and that there are no thick aquifers in the investigated area. In addition to that the shaly sand sediments have iron rich clays, which pollute groundwater, deeper resistive layers formed by conglomerates are considered the main aquifers of this area.

Search for anomalous effects in geophysical fields in relation to earthquakes in the Baikal rift zone

The paper presents data of electrical conductivity monitoring for the upper crust obtained with the use of a controllable electromagnetic source by the dipole, vertical electromagnetic, and transient electromagnetic sounding methods, as well as variations in the geomagnetic field and the Lake Baikal water level. With the help of spectral, spectral time, and multivariate analyses, main spatial and temporal patterns of variations in geophysical parameters are determined. The present study revealed anomalous effects in the behavior of the canonical coherence (parameter K) and aggregated signal (parameter A) and other anomalous variations in geophysical parameters related to earthquakes in the Baikal rift zone.

Application of the adaptive shrinkage genetic algorithm in the feasible region to TEM conductive thin layer inversion

Combining the adaptive shrinkage genetic algorithm in the feasible region with the imaging of apparent vertical conductance differential, we have inverted the TEM conductive thin layer. The result of the inversion demonstrates that by adaptive shrinkage in the feasible region, the calculation speed accelerates and the calculation precision improves. To a certain extent, in this method we surmount the transient electromagnetic sounding equivalence and reduced equivalence scope. Comparison of the inverted result with the forward curve clearly shows that we can image the conductive thin layer.

Contributions to the geological mapping of Mors, Denmark – A study based on a large-scale TEM survey

Recent improvements of the transient electromagnetic (TEM) method provide unprecedented capabilities of imaging geological features in the uppermost few hundred metres of the subsurface. This is documented with an example from the island of Mors, Northern Jutland, Denmark, where 2655 TEM soundings have been acquired as a part of Viborg County’s hydrogeological investigation programme. Horizontal aspects of the interpreted TEM soundings are presented in thematic maps revealing variations of resistivities in specified depth intervals and depths to the resistivity basement, i.e. the deepest recorded low-resistive layer. Vertical aspects are displayed in cross-sections.The dense coverage of TEM soundings, combined with borehole logs, allows the precise delineation of major geological features. These comprise the Mors salt diapir, intricate systems of buried Quaternary valleys and several glaciotectonic complexes, all contributing to a complicated geological framework for the island. The layers above the salt diapir, which have been subject to severe erosion, are distinctly imaged in the thematic maps, even though they are frequently dissected by buried valleys. At least four generations of buried valleys can be identified on the basis of their preferred orientations; these were mainly formed during early glaciations. Large parts of the island were glaciotectonically deformed during the Late Weichselian subsequent to valley formation, and it is suggested that the presence of deep valleys may have affected the process of glaciotectonic deformation.

Geophysical investigations of buried Quaternary valleys in Denmark: an integrated application of transient electromagnetic soundings, reflection seismic surveys and exploratory drillings

Buried Quaternary valleys are important hydrological structures in Denmark. Geophysical and geological investigations were performed to develop an integrated interpretational methodology for a quantitative description of their structure and lithology. Three buried valleys in central eastern Jutland, Denmark were investigated using the transient electromagnetic (TEM) sounding method, two-dimensional reflection seismic profiling, vertical seismic profiling (VSP) and analyses of data and samples from exploratory drillings. The most advantageous approach for fieldwork is the sequence of (1) resistivity mapping with TEM to identify the buried valleys, (2) reflection seismic profiling across the valleys, (3) location and drilling of exploratory drillings, and (4) vertical seismic profiling. The accuracy of the geological interpretation is improved substantially by combining the structural information from the seismic profiling with the predominantly lithological information derived from the TEM soundings. Sequential interpretation of the data sets is optimal when variations in geology are low to moderate. Strong variations may, however, lead to breakdown of the interpretation models. Buried valleys occur primarily as cut-and-fill structures filled with Quaternary deposits consisting of till, glacio-lacustrine clay/silt and meltwater sand and gravel. The seismic velocity of the tills is considerably higher (2150 m/s) than it is for the other valley-fill sediments (1750 m/s). Resistivities of the clays are distinctly less than those of sands and gravels. Hence, classification of these parameters gives indirect information about the lithology of the valley fill as well as the structure.

Imaging buried Quaternary valleys using the transient electromagnetic method

Buried Quaternary valleys in Denmark are complex structures filled with various deposits consisting primarily of glacio-lacustrine clay, till and meltwater sand, and gravel. The valleys are important geophysical targets, because they often contain significant volumes of groundwater used for public water supply. About 700 km of buried valley structures have been imaged in the western part of Denmark by the transient electromagnetic (TEM) method. The ability to map the valleys depends primarily on valley geometry, infill architecture and the resistivity of the fill sediments as well as the substratum. One-dimensional (1-D) inversion models of the TEM soundings have been used to construct contour maps of 20 m average resistivities and depth to a good conductor, which provide images for geological interpretation. Images of buried valley morphology, fill properties, infill architecture, such as cut-and-fill structures, valley distribution and valley generations, are characterized for case studies from Hornsyld, Holstebro and the Vonsild/Agtrup areas of Denmark.

Ground-based geophysical measurements in salinity-affected areas prior to AEM surveys

Ground-based electromagnetic and electrical surveys were conducted in areas of southeast Queensland in March 2001 to establish representative ground conductivity values down to depths of greater than 50 m for use in designing suitable airborne electromagnetic (AEM) surveys to be flown in those areas later that year. Once AEM data were available, a comparison between the results of the ground-based and AEM surveys was made. High resolution definition of near-surface conductive structures was achieved with the combined use of DC resistivity and frequency-domain EM31 measurements, while transient electromagnetic (TEM) soundings provided reliable values of deeper conductivity. A comparison of the results of ground-based and corresponding AEM data shows that there is broad agreement between the behaviour of the conductivity of the conductive layer detected by AEM and corresponding conductivity values derived from ground-based data. However, the models obtained from the two sets of data do not agree in detail, probably because the AEM inversions were constrained to have a resistive basement, whereas ground-based and down-hole results show that the basement is generally relatively conductive.

Large-scale TEM investigation for groundwater

This abstract outlines the use of the transient electromagnetic (TEM) method for hydrogeophysical investigations. The TEM method has proven to be a strong tool for the delineation of water bearing sand and gravel layers. We discuss newly developed TEM systems (PATEM and HiTEM) along with developments in data interpretation. Finally the TEM part of the Aarhus Survey is presented. This survey covers an area of more than 100 km2 and about 6000 TEM soundings have been measured.

Large-scale TEM investigation for groundwater

The transient electromagnetic (TEM) method has proven to be a powerful tool for the delineation of water-bearing sand and gravel layers in hydrogeophysical investigations. Advancements in the technique include newly developed TEM systems—the Pulled Array TEM (PATEM) and the High Moment TEM (HiTEM) systems—along with innovations in data interpretation. The advanced methodology is illustrated in two case histories. The first, a large-scale TEM survey northwest of Aarhus, Denmark, includes more than 2700 conventional 40 m × 40 m TEM soundings and approximately 500 km of PATEM data. Thematic maps of interval resistivities and depth to low-resistivity Palaeogene clays provide a detailed view of the hydrogeological conditions in the area. The second is a groundwater investigation at Hedensted, Denmark. Based on the hydrogeophysical survey, a new well field in this area was located and developed. The initial phase of the survey included 500 TEM soundings. Based on the interpretation of the TEM survey, two seismic sections were acquired across formerly unknown aquifers. The seismic data made it possible to select optimal locations for three wells that were drilled subsequently. The new well field yields up to 400 000 m3 water/year.

A joint TEM-HLEM geophysical approach to borehole sitting in deeply weathered granitic terrains.

The accurate location of aquiferous fracture zones in granite beneath a > 50 m thick weathered mantle in semi-arid regions is a major hydrogeological problem. It is expected that the zone of intensive fracturing will be more susceptible to weathering and thus be characterized by the thickest development of saprolite, a good electrically conductive target for deep-probing electromagnetic systems. The single-loop transient electromagnetic (TEM) technique is well known to have the capability for detecting concealed steep mineralized targets in mining environments and can be adapted to this hydrogeological problem. We propose that combining the conventional frequency-domain horizontal-loop electromagnetic (HLEM) and single-loop TEM is an effective practical approach to locating concealed aquiferous fracture zones. In the supporting case studies presented here, we deployed multifrequency HLEM profiling (with 50 m transmitter-receiver separation) and TEM soundings with contiguous 10 or 20 m sided loops along the survey lines in a granitic terrain affected by deep (> 50 m) weathering in northeast Brazil. A somewhat layered structure consisting of resistive hardpan/leached zone, conductive saprolite, and resistive basement is identifiable in the typical TEM depth sounding data. We obtained coincident HLEM and TEM anomalies at all the sites, enabling a relatively straightforward selection of potential drilling positions. Simple resistivity-depth transformation of the TEM data was done for each site, yielding an approximate section from which drilling depths were estimated. All of the boreholes located were successful. Although our results appear to indicate that the single-loop TEM method could be used independently for borehole sitting in deeply weathered granitic terrains and that the weathering profile over granite can be mapped using TEM depth soundings of appropriate observational bandwidth, we recommend a joint electromagnetic approach for optimal well sitting.

3-D approximate inversion for transient electromagnetic soundings

In this paper, based on the theories of smoke ring, current lines in the Earth, induced by transmitting source on the Earth’s surface, are regarded as downward moving and outward expanding current filaments which are called as images of transmitting source. By comparing the magnetic field of the current filaments with that measured near the source, we can determine the image depth which can be used for estimating the downward vertical velocity of the image by using a cubic spline interpolation method. Then the resistivities varying with depth can be approximately estimated through a comparison between the estimated velocity of the image and the downward diffusing velocity in a homogeneous half space. We discussed the case of a circular transmitting loop, and developed a corresponding approximate inversion program which has been tested for 1-D layered-Earth models and real data.

Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion)

A study of the geoelectrical structure of the central part of Piton de la Fournaise volcano (Reunion, Indian Ocean) was made using direct current electrical (DC) and transient electromagnetic soundings (TEM). Piton de la Fournaise is a highly active oceanic basaltic shield and has been active for more than half a million years. Joint interpretation of the DC and TEM data allows us to obtain reliable 1D models of the resistivity distribution. The depth of investigation is of the order of 1.5 km but varies with the resistivity pattern encountered at each sounding. Two-dimensional resistivity cross sections were constructed by interpolation between the soundings of the 1D interpreted models. Conductors with resistivities less than 100 ohm-m are present at depth beneath all of the soundings and are located high in the volcanic edifice at elevations between 2000 and 1200 m. The deepest conductor has a resistivity less than 20 ohm-m for soundings located inside the Enclos and less than 60–100 ohm-m for soundings outside the Enclos. From the resistivity distributions, two zones are distinguished: (a) the central zone of the Enclos; and (b) the outer zone beyond the Enclos. Beneath the highly active summit area, the conductor rises to within a few hundred meters of the surface. This bulge coincides with a 2000-mV self-potential anomaly. Low-resistivity zones are inferred to show the presence of a hydrothermal system where alteration by steam and hot water has lowered the resistivity of the rocks. Farther from the summit, but inside the Enclos, the depth to the conductive layers increases to approximately 1 km and is inferred to be a deepening of the hydrothermally altered zone. Outside of the Enclos, the nature of the deep, conductive layers is not established. The observed resistivities suggest the presence of hydrated minerals, which could be found in landslide breccias, in hydrothermally altered zones, or in thick pyroclastic layers. Such formations often create perched water tables. The known occurrence of large eastward-moving landslides in the evolution of Piton de la Fournaise strongly suggests that large volumes of breccias should exist in the interior of the volcano; however, extensive breccia deposits are not observed at the bottom of the deep valleys that incise the volcano to elevations lower than those determined for the top of the conductors. The presence of the center of Piton de la Fournaise beneath the Plaine des Sables area during earlier volcanic stages (ca. 0.5 to 0.150 Ma) may have resulted in broad hydrothermal alteration of this zone. However, this interpretation cannot account for the low resistivities in peripheral zones. It is not presently possible to discriminate between these general interpretations. In addition, the nature of the deep conductors may be different in each zone. Whatever the geologic nature of these conductive layers, their presence indicates a major change of lithology at depth, unexpected for a shield volcano such as Piton de la Fournaise.

Evaluation of small-loop transient electromagnetic soundings to locate the Sherwood Sandstone aquifer and confining formations at well sites in the Vale of York, England

Shallow-depth transient electromagnetic (TEM) soundings have been performed at six borehole locations in an intensively farmed area in northern England to evaluate their usefulness in mapping geological formations under a thick cover of glacial drift deposits. The regionally important Triassic Sherwood Sandstone (SS) Group aquifer is directly overlain by Triassic Mercia Mudstone in the eastern two-thirds of the study area and by drift deposits in the west. Owing to the difficulty of deploying large loops and the overriding need to minimize lateral effects on the depth probes, square transmitter loops of 20, 40 and 50-m side-lengths were deployed in the central-loop configuration with the Geonics EM47 and PROTEM47/57 field equipment. Using a two-stage data interpretation technique, it is found that the effective depth of mapping ranged from about 8 to 150 m at most sounding locations. Comparison of inversion models with borehole data shows that the SS and some overlying sedimentary rocks may be discerned from the TEM soundings; there is a consistent pattern of resistivity distribution within each geological formation at all the borehole sites enabling a realistic identification of the key stratal units. However, a 7–11-m-thick upper layer is found in all the constructed models, which does not correlate with any known formation boundaries, but appears to be justified by comparison with sample dc resistivity soundings at two locations; it would also appear that the earliest time windows (<0.016 ms) are somewhat distorted by the band-limitation operation of the TEM instrumentation. This pilot study demonstrates that the TEM method is a potent tool for stratigraphic mapping in the region, but the upper 5–8 m remains largely inaccessible to the method using state-of-the-art equipment and conventional data processing techniques. It may therefore be necessary to combine TEM and short spread-length ( AB/2≤25 m) dc resistivity depth soundings to accurately map the near-surface in this glacial-covered terrain.

TEM Investigations of Aquifers in the Southwest Coast of Taiwan

AbstractA large data set of more than 330 transient electromagnetic (TEM) soundings was collected in the southwest coast of Taiwan. The aims of the study were (1) to define the shallow geoelectrical structure, and (2) locate large bodies of ground water by integrating electrical anomalies and data from more than 140 wells. A one‐dimensional, smooth‐model inversion of TEM data was used, and trend surface analysis of those results was employed to refine the results for interpretation. A resistive zone (15 to 160 ohm‐m resistivity) at elevations 20 to 120 m below sea level in the northern part of the survey area is interpreted as fresh water saturated sand or gravel. The southern part has extremely low resistivity, less than 1 ohm‐m, indicating a widespread sea water intrusion. The divide between these two parts is located at about the middle of the study area. This study demonstrates that TEM investigation can help resolve ambiguities, even in an area with many wells, and thereby increase the likelihood of success in ground water exploration.

Inversion of band‐limited TEM responses

It is shown that the electromagnetic (EM) spectrum is characterized by strong amplitude‐modulated transmitters operating in the target bandwidth of transient electromagnetic (TEM) measurements. As these transmitters cause significant noise in TEM soundings, it is mandatory to band‐limit the input signals to improve the signal‐to‐noise ratio and thereby the depth of exploration. Band‐limitation will distort the TEM responses, which leads to erroneous inversion results if the applied low‐pass filters are not accounted for in the inversion scheme. We incorporate the low‐pass filters in the inversion scheme and test the inversion approach on theoretical and field data. Inversion of band‐limited theoretical responses results in recovery of erroneous resistivity models if the filters are not included in the inversion scheme. By contrast, inversion of band‐limited theoretical and field data, for which the applied low‐pass filters are included in the inversion scheme, leads to recovery of similar resistivity models, independent of the applied cut‐off frequencies.

Geophysical investigation of proposed sites for artificial recharge in Northern Oman

Abstract Sites in the interior of the Sultanate of Oman have been considered for the construction of recharge dams for temporarily impounding flood flows from the Northern Oman Mountains. Impounded water would be discharged under control to infiltrate into downstream aquifers. Four geophysical methods were applied to characterization of the spreading ground downstream of these sites and to guide drilling of test bores. The main aquifers occur in permeable zones at the base of the alluvium and in the upper, weathered, bedrock. The alluvium is variably cemented and clay rich, and behaves as a confining layer between the shallow, unconsolidated surface infiltration zone and the aquifer. The thickness of the upper unconsolidated layer of boulders and gravels was most accurately mapped with seismic refraction or vertical electrical sounding (VES). These methods were slow to apply in the arid, bouldery ground conditions and surveying was in places constrained by the wadi walls. Gravity survey, with control provided by VES, was a rapid method of profiling the total thickness of alluvium and weathered zone overlying the denser and more resistive unweathered ophiolite bedrock. VES was usefully supplemented by simultaneously measured induced polarization (IP) soundings which respond to the distribution of clay minerals. Transient electromagnetic sounding (TEM) proved to be the most rapid method of survey as no ground contact was required. Alluvial thickness was interpreted by resistivity-depth modelling of TEM data and was most reliable over low-resistivity argillaceous mudstone or shale bedrock.

The main structure and tectonic features of the Chernorud-Mukhor site on the western shore of Lake Baikal from TEM and SP measurements

Transient electromagnetic (TEM), self-potential (SP) and geoelectrical mapping measurements were carried out at the Chernorud-Mukhor site in the Priolkhonje area on the western shore of the Lake Baikal. All measurements were made along several profiles across the main strike of the regional Primorsky fault. TEM measurements were carried out in a time range from a few tens of microseconds to several tens of milliseconds. The most important result of the 1D modelling of TEM soundings is the discovery of nearly horizontal boundaries that divide high resistive overlying and well conducting underlying rocks. The resistivity of the former is in the range from 100 Ωm to 1000 Ωm, while the resistivity of the latter varies from less than 1 Ωm to several tens of Ωm. This good conductive zone could also be verified by geoelectrical mapping using Schlumberger array ( AB/2=100 m). Due to high conductivity of the underlying rocks only the upper boundary of the conductive layer could be determined by TEM soundings. A regional SP anomaly with amplitude of about −450 mV has also been observed above the low resistivity zone indicating the electron nature of its conductance. Geologically, the conductive zone is represented by a graphite-bearing layer within the region of archean rocks. Since that layer extends over a large area, it may be used as a key in studying structures and tectonics of the Priolkhonje area. A 1D TEM geoelectric section shows a wide, gently sloping syncline as a probable base structure of the Chernorud-Mukhor site. Neotectonic faults divide the syncline into vertically displaced blocks that form a wide complicated graben with a total amplitude of about 250 m.

Minesite groundwater contamination mapping

A number of electromagnetic and electrical methods have been applied jointly to map groundwater contamination near minesite landforms. The main methods being investigated are transient electromagnetic (TEM) and direct current (DC) resistivity soundings, DC profiling, self potential (SP), and induced polarisation (IP) measurements, combined with any existing hydrogeological and hydrogeochemical data.At the tailings dam of the disused Brukunga pyrite mine in South Australia, reaction of groundwater with the tailings causes the formation and discharge of sulphuric acid. Geophysical methods have been investigated to determine whether they can be used to characterise variations in depth to watertable and map preferred groundwater flow paths through the tailings dam. The results of the geophysical surveys show that it is difficult to determine any preferred channels of groundwater flow from SP profiling data alone, but TEM and DC sounding measurements have enabled accurate determination of watertable levels and aquifer resistivity. The shallowest and most resistive part of the aquifer occurs in the southeast of the site, and we deduce that a possible source of fresh groundwater entering the site occurs here. It has been recommended that efforts to reduce acid formation in the tailings dam should concentrate on reducing this inflow of groundwater.

Applications of TEM in Taiwan

Based on the results of a large set of field data from Sirotem transient electromagnetic (TEM) soundings carried out in Taiwan, three case studies demonstrate the applications of TEM in Taiwan. The transmitter-receiver array used was the in-loop configuration with the dimensions of the square transmitter loops varying from 20 m to 400 m on a side for effective exploration depths of 20 m to 500 m. The Occam’s smooth model inversion was employed to reduce the temptation to over interpret the data and to eliminate arbitrary discontinuities in simple layered models. The inverted results were filtered and then organised into both depth slice and cross sections to study the 3D geoelectric structures of the survey area.The first case study is located on the southwestern coast of Taiwan. The 3D resistivity images show fresh-water-saturated sand and/or gravel sediments in the northern part of the survey area, while a widespread sea-water intrusion is evident in the South. These findings are supported by more than 140 wells.The second case study is located in eastern Taiwan. Based on the TEM sounding results, one predominant electrical discontinuity can be recognised in the Longitudinal Valley that agrees well with the known suture trace on the surface between the Philippine Sea and the Eurasian plates.The third case study is in the urbanised Taipei Basin. The 3D resistivity images display basement topography and trace the probable faults of the Taipei Basin. These findings are supported by more than 180 wells. Therefore, TEM surveys can work around urbanised cities, where ambient electrical noise is high, and open spaces limited.

A comparative analysis between transient electromagnetic soundings and resistivity soundings in the Tres Virgenes geothermal zone, Mexico

Las zonas geotérmicas en áreas volcánicas están caracterizadas por relieves topográficos abruptos y resistividades superficialesvariables, representadas por manantiales termales, zonas fósiles y actuales de alteración hidrotermal y la presencia de diferentesepisodios de volcanismo explosivo y lávico. En este trabajo examinamos los resultados de la aplicación de sondeos electromagnéticostransitorios (TEM) de bobina central y de sondeos eléctricos verticales (SEV) en una zona geotérmica con estas características. Lainversión de 62 SEV y 55 sondeos TEM da como resultado modelos estratificados promedio similares. Sin embargo, la evaluaciónde los modelos usando el método de descomposición en valores singulares demuestra que los sondeos TEM tienen un mayor poderresolutivo. Los factores que contribuyen al mejor desempeño del método TEM son: una mayor redundancia de datos, una mejorcalidad de los datos, una menor sensibilidad a la topografía y a heterogeneidades superficiales y una menor sensibilidad a problemasde equivalencia asociados con capas conductoras delgadas.