Time-domain Electromagnetic Data Research Articles

A New Imaging Approach for Dipole–Dipole Time-Domain Electromagnetic Data Based on the q-Transform

The inline dipole–dipole time-domain electromagnetic method is a useful supplementary tool for the exploration of a resistive target such as a hydrocarbon resource. Currently, the interpretation of the data from this configuration is based on the fitting of the modeled response and the recorded response in a least square sense. In the paper, we propose an imaging method based on a mathematical transformation named the q-transform, which could make the subsequent imaging of TEM data using the interpretation technique in seismic method possible. The imaging strategy is based on analytical derivation and analysis of the transformed wave-field from which pseudo-velocity and resistivity can be extracted. A numerical test is employed to get the transformed wave-field from the time-domain electromagnetic response with the selected regularization method. The proposed imaging method is tested on the synthetic data obtained from a homogeneous and layered half space models. The results reveal that it is a useful method to recover the resistivity of an underground medium and can provide an additional tool for fast and qualitative imaging of the subsurface resistivity.

Inductive induced polarization effect in heliborne time-domain electromagnetic data for uranium exploration, northern part of Cuddapah Basin, India

The Peddagattu, Lambapur, Chitrial and Koppunuru uranium deposits along the northern margins of the Cuddapah Basin are confined to the middle Proterozoic unconformity interface between Archean basement granites and the overlying resistive quartzites. Negative transients observed in the coincident loop heliborne time-domain electromagnetic (HTEM) data over these deposits (occurring in outliers) are believed to be due to thick polarizable conductive zones occurring along the unconformity. Similar negative HTEM responses are recorded over the Gorukunta Tanda outlier. A ground spectral induced polarization (SIP) survey conducted over the outlier and ground geologic observations indicated an altered basement/regolith with thickness up to 5 m below 20–30 m thick quartzite. Interpretation of Cole-Cole parameters computed from the SIP data indicated a change in the dispersion. These Cole-Cole parameters were used in modeling negative HTEM data assuming a polarizable plate placed in a layered earth at a depth of approximately 50 m using the CSIRO LeroiAir program. A negative [Formula: see text] response in the late channels indicated that the negatives can be explained in terms of inductive induced polarization effects. Modeling of HTEM [Formula: see text] data for the profile through the Lambapur uranium deposit and the Gorukunta Tanda reveals the presence of a polarizable lithologic unit at a depth of approximately 40 m. This unit is interpreted as an argillic alteration of basement, with the presence of clay and/or disseminated sulfides as evidenced from the core extracted from the boreholes at depths below the highly resistive quartzite. Uranium mineralization is closely associated with altered basement and sulfides along the unconformity where the distinct negative electromagnetic (EM) signature is recorded. Furthermore, there exists a good correlation between the uranium mineralization grade and the thickness versus the averaged late-channel negative HTEM response over the known deposits. The negative EM response helped in locating the new target areas for uranium exploration.

Central Loop Time Domain Electromagnetic Inversion Based on Born Approximation and Levenberg-Marquardt Algorithm

Time Domain Electromagnetic (TDEM) method uses electromagnetic wave to detect resistivity or conductivity difference of lithology in the subsurface, measured in the time domain. TDEM method has been developed in decades. There are forward modeling and inversion programs have been made. The purpose of making the forward modeling programs is to calculate TDEM response so that data acquisition parameters can be chosen correctly. The inversion program is for synthesizing geological model from measured TDEM data. Several TDEM programs have been made, but the procedures require heavy and complex computation, which need high computer specification and lot of calculation time. Nowadays program that used less complex computation and faster calculation is needed to match field data acquisition productivity. To achieve faster and more accurate process, we use Born Approximation of the apparent conductivity for the forward modeling program and Levenberg-Marquardt algorithm for the inversion program. We use many circumstances in testing our programs, from one layer to multi layers by varying the resistivity or thickness of lithology and compared with a validated program, EMUPLUS. At the end of the test, inversion of real data is taken as confirmation that the program can be used to process real TDEM data. Inversion results of both synthetic and real TDEM data show pleasant results. These test results indicate that our program can be used as daily forward modeling program for determining data field acquisition parameters and do the inversion procedure for synthetic and real TDEM data.

Modeling of Floating Time Domain Electromagnetic Method to Detect Dissolved Sediment

In hydrology context, sediment can be interpreted as inorganic and organic material that is transported by, suspended in, or deposited by streams. It is important to know the function of soil, stream discharge, land-cover features, weather conditions and land-use activities. Sediment load carried by streams and rivers can be composed either of fine materials, mostly silts, and clays, or coarse materials such as sand. One product of sediment is dissolved load consists of indistinct material in solution moving downstream. It is produced by chemical weathering processes and does not move out of the water. To investigate the dissolved sediment, we have applied the floating of Time Domain Electromagnetic (TDEM) method. The acquisition of TDEM data has been performed use tires and small ship as innovation measurements. The calculated data model using Occam and Marquardt Algorithms. The responses of data show the sedimentation has less resistive compare the surrounding structures. This innovation is very helpful to know the environmental condition, especially in the water.

Apparent conductivity-depth estimation of fixed-wing time-domain electromagnetic two-component data based on iterative lookup tables

Apparent conductivity-depth estimation of airborne electromagnetic data is generally applied to identify conductive targets. As the attitude and height of the aircraft change during a flight survey, the collected data and corresponding apparent conductivity-depth results can be affected. An improved apparent conductivity-depth algorithm with consideration of height and pitch is developed herein based on an iterative lookup table. Two sets of tables of Bx−Bz−apparentconductivity are established in association with the two parameters of pitch and height. The apparent conductivity result is achieved by looking up two tables iteratively, as the pitch and height parameters are obtained from looking up one table and are used in looking up the other table. The iterative looking up ends when the apparent conductivity and the two parameters satisfy the desired precision. The depth is derived from the apparent conductivity and channel delay time. Tests that employ synthetic data demonstrate the feasibility of this iterative lookup table method.

Modeling induced polarization effects in helicopter time domain electromagnetic data: Synthetic case studies

We have developed a synthetic multiparametric modeling and inversion exercise undertaken to study the robustness of inverting airborne time-domain electromagnetic (TDEM) data to extract Cole-Cole parameters. The following issues were addressed: nonuniqueness, ill posedness, dependency on manual processing and the effect of constraints, and a priori information. We have used a 1D layered earth model approximation and lateral constraints. Synthetic simulations were performed for several models and the corresponding Cole-Cole parameters. The possibility to recover these models by means of laterally constrained multiparametric inversion was evaluated, including recovery of chargeability distributions from shallow and deep targets based on analysis of induced polarization (IP) effects, simulated in airborne TDEM data. Different scenarios were studied, including chargeable targets associated with the conductive and resistive environments. In particular, four generic models were considered for the exercise: a sulfide model, a kimberlite model, and two generic models focusing on the depth of investigation. Our study indicated that, in cases when relaxation time ([Formula: see text]) values are in the range to which the airborne electromagnetic is most sensitive (e.g., approximately 1 ms), it is possible to recover deep chargeable targets (to depths more than 130 m) in association with high electrical conductivity and in resistive environments. Furthermore, it was found that the recovery of a deep conductor, masked by a shallower chargeable target, became possible only when full Cole-Cole modeling was used in the inversion. Lateral constraints improved the recoverability of model parameters. Finally, modeling IP effects increased the accuracy of recovered electrical resistivity models.

Modeling induced polarization effects in helicopter time-domain electromagnetic data: Field case studies

Induced polarization (IP) effects are becoming more evident in time-domain helicopter airborne electromagnetic (AEM) data thanks to advances in instrumentation, mainly due to improvements in the signal-to-noise ratio and hence better data quality. Although the IP effects are often manifested as negative receiver voltage values, which are easy to detect, in some cases, IP effects can distort recovered transients in other ways so they may be less obvious and require careful data analysis and processing. These effects represent a challenge for modeling and inversion of the AEM data. For proper modeling of electromagnetic transients, the chargeability of the subsurface and other parameters describing the dispersion also need to be taken into consideration. We use the Cole-Cole model to characterize the dispersion and for modeling of the IP effects in field AEM data, collected by different airborne systems over different geologies and exploration targets, including examples from diamond, gold, and base metal exploration. We determined how multiparametric inversion techniques can simultaneously recover all four Cole-Cole parameters, including resistivity [Formula: see text], chargeability [Formula: see text], relaxation time [Formula: see text], and frequency parameter [Formula: see text]. The results obtained are in good agreement with the ancillary information available. Interpretation of the IP effects in AEM data is therefore seen by the authors as providing corrected electrical resistivity distributions, as well as additional information that could assist in mineral exploration.

Airborne IP: examples from the Mount Milligan deposit, Canada, and the Amakinskaya kimberlite pipe, Russia

There have been multiple occurrences in the literature in the past several years of what has been referred to as the induced polarisation (IP) effect in airborne time domain electromagnetic (TDEM) data. This phenomenon is known to be responsible for incorrect inversion modelling of electrical resistivity, lower interpreted depth of investigation (DOI) and lost information about chargeability of the subsurface and other valuable parameters. Historically, there have been many suggestions to account for the IP effect using the Cole-Cole model. It has been previously demonstrated that the Cole-Cole model can be effective in modelling synthetic TDEM transients. In the current paper we show the possibility of extracting IP information from airborne TDEM data using this same concept, including inverse modelling of chargeability from TDEM data collected by VTEM, with field examples from Canada (Mt Milligan deposit) and Russia (Amakinskaya kimberlite pipe).

Joint inversions of two VTEM surveys using quasi-3D TDEM and 3D magnetic inversion algorithms

In the current paper, we present results of a joint quasi-three-dimensional (quasi-3D) inversion of two versatile time domain electromagnetic (VTEM) datasets, as well as a joint 3D inversion of associated aeromagnetic datasets, from two surveys flown six years apart from one another (2007 and 2013) over a volcanogenic massive sulphide gold (VMS-Au) prospect in northern Ontario, Canada. The time domain electromagnetic (TDEM) data were inverted jointly using the spatially constrained inversion (SCI) approach. In order to increase the coherency in the model space, a calibration parameter was added. This was followed by a joint inversion of the total magnetic intensity (TMI) data extracted from the two surveys. The results of the inversions have been studied and matched with the known geology, adding some new valuable information to the ongoing mineral exploration initiative.

Integrated electromagnetic data investigation of a Mesozoic CO2 storage target reservoir-cap-rock succession, Svalbard

Recently acquired time-domain electromagnetic (TEM) and magnetotelluric (MT) data sets are utilized in the first electromagnetic (EM) characterization of a geological CO2 storage target site in Adventdalen, Arctic Norway. Combining the two EM data sets enabled to resolve the electrical resistivity structure of the target site better than either of the methods alone. 2D inverting the MT data in the audio period interval (0.003–1s) with supporting input derived from the TEM data (0.01–10ms) provided a geologically meaningful resistivity model that included information not previously evident from existing seismic and borehole data. The ca. 1.8×1km 2D resistivity model displays a laterally constrained highly conductive anomaly (ca. 10Ωm) at about 400–500m depth, where reflectors of a parallel seismic section are concealed and core samples indicate a highly fractured décollement zone formed during Paleogene compression. The base of the permafrost is imaged at ca. 200m depth. Synthetic inversion tests, however, suggest that this may be exaggerated by tens of meters, due to a thin conductive layer present approximately between 10 and 25m depth. The resistivity model does not give indication for a fluid pathway we can connect to leakage, in line with water injection and leak-off tests in the reservoir and cap-rock, both of which indicate a sealing shale-dominated cap-rock separating an over-pressured compartment above the sealing shale from a severely under-pressured reservoir interval. The results we present indicate the advantage of integrating EM exploration techniques in a CO2 reservoir-cap-rock study to obtain a more complete picture.

Combining electromagnetic measurements in the Mygdonian sedimentary basin, Greece

We present a novel approach where time-domain electromagnetic (TEM) data are transformed and subsequently used in two-dimensional (2-D) magnetotelluric inversion of the determinant of the impedance tensor. The main idea is to integrate TEM with magnetotelluric (MT) data to produce subsurface electrical resistivity models. Specifically, we show that 2-D MT data inversion of the determinant of the impedance tensor supported by inclusion of TEM–MT-transformed data has superior resolution at the near surface and at the same time static shift afflicting the MT data can be addressed. Thus, the approach allows for practical express integration of TEM data with MT measurements as opposed to a full combined 3-D inversion, which requires significant resources. The approach is successfully applied in the Mygdonian sedimentary basin located in Northern Greece. In addition to TEM and MT data, also controlled source — and radiomagnetotelluric data are available from the Mygdonian basin, which have been subjected to 2-D analysis previously. We have extended the analysis to a full 3-D inversion using ModEM code. All obtained models are analysed and are in a good agreement.

On recovering distributed IP information from inductive source time domain electromagnetic data

On recovering distributed IP information from inductive source time domain electromagnetic data

Noise reduction of time domain electromagnetic data: Application of a combined wavelet denoising method

AbstractA denoising method based on wavelet analysis is presented for the removal of noise (background noise and random spike) from time domain electromagnetic (TEM) data. This method includes two signal processing technologies: wavelet threshold method and stationary wavelet transform. First, wavelet threshold method is used for the removal of background noise from TEM data. Then, the data are divided into a series of details and approximations by using stationary wavelet transform. The random spike in details is identified by zero reference data and adaptive energy detector. Next, the corresponding details are processed to suppress the random spike. The denoised TEM data are reconstructed via inverse stationary wavelet transform using the processed details at each level and the approximations at the highest level. The proposed method has been verified using a synthetic TEM data, the signal‐to‐noise ratio of synthetic TEM data is increased from 10.97 dB to 24.37 dB at last. This method is also applied to the noise suppression of the field data which were collected at Hengsha island, China. The section image results shown that the noise is suppressed effectively and the resolution of the deep anomaly is obviously improved.

Integrated geophysical application to investigate groundwater potentiality of the shallow Nubian aquifer at northern Kharga, Western Desert, Egypt

Continuous evaluation of groundwater aquifers in the basin of Kharga Oasis is very important. Groundwater in Kharga Oasis represents the major factor for the development plans of this area as it is the sole source for water supplies required for drinking and irrigation purposes. This study is concerned by analyzing the groundwater potentiality of the shallow aquifer at the northern part of Kharga basin by integrated application of Vertical Electrical Sounding (VES) and Time domain Electromagnetic (TEM) techniques. The VES data were measured at 28 points arranged along a north–south trending line by applying Schlumberger array with a maximum current-electrode spacing (AB) of 1000m. The TEM data were measured at 167 points arranged along 11 east–west trending lines by using a single square loop with 50m loop-side length. The VES and TEM data have been individually inverted, where the VES models were used as initial models for TEM data inversion. The final models were used for construction of 17 geoelectrical sections and 5 contour maps describing subsurface water-bearing layers at the investigated area. Correlation of the obtained models with geologic, hydrogeologic and borehole information indicates that the shallow aquifer comprises two zones (A-up) and (B-down) separated by a highly conductive shale layer. The upper zone (A) is composed of fine to medium sand with thin clay intercalations. It exhibits low to moderate resistivities. This zone was detected at depth values ranging from 10 to 70m below ground surface (bgs) and shows a thickness of 25–90m. The lower zone (B) exhibits moderate to high resistivity values with expected good water quality. The upper surface of zone B was detected at 60–165m depth.

Bedrock mapping of buried valley networks using seismic reflection and airborne electromagnetic data

In glaciated terrain, buried valleys often host aquifers that are significant groundwater resources. However, given the range of scales, spatial complexity and depth of burial, buried valleys often remain undetected or insufficiently mapped. Accurate and thorough mapping of bedrock topography is a crucial step in detecting and delineating buried valleys and understanding formative valley processes. We develop a bedrock mapping procedure supported by the combination of seismic reflection data and helicopter time-domain electromagnetic data with water well records for the Spiritwood buried valley aquifer system in Manitoba, Canada. The limited spatial density of water well bedrock observations precludes complete depiction of the buried valley bedrock topography and renders the water well records alone inadequate for accurate hydrogeological model building. Instead, we leverage the complementary strengths of seismic reflection and airborne electromagnetic data for accurate local detection of the sediment-bedrock interface and for spatially extensive coverage, respectively. Seismic reflection data are used to define buried valley morphology in cross-section beneath survey lines distributed over a regional area. A 3D model of electrical conductivity is derived from inversion of the airborne electromagnetic data and used to extrapolate buried valley morphology over the entire survey area. A spatially variable assignment of the electrical conductivity at the bedrock surface is applied to different features of the buried valley morphology identified in the seismic cross-sections. Electrical conductivity is then used to guide construction of buried valley shapes between seismic sections. The 3D locus of points defining each morphological valley feature is constructed using a path optimization routine that utilizes deviation from the assigned electrical conductivities as the cost function. Our resulting map represents a bedrock surface of unprecedented detail with more complexity than has been suggested by previous investigations. Our procedure is largely data-driven with an adaptable degree of expert user input that provides a clear protocol for incorporating different types of geophysical data into the bedrock mapping procedure.

A Magnetotelluric survey of the North Perth Basin: A technical case study

SUMMARY Original motivation of this study was to understand important structures with a proven geothermal signature associated with high temperatures in the shallower basin and high flow rates in the aquifers. Anomalous temperatures are recorded around the Beagle Ridge and significant flow rates observed near the Urella Fault, factors important to unconventional geothermal prospects. The focus of this study was therefore a detailed geophysical investigation of several Geothermal Exploration Permits (GEPs) in the North Perth Basin. Two Magnetotelluric (MT) surveys are conducted over target areas in the North Perth Basin and adjoining tectonic domains to provide information about the electrical conductivity regime of the basin and western margin of the Yilgarn Craton. Existing geophysical data in this part of the basin are sparse and electrical data for the basin in general is limited to shallow Time Domain Electromagnetic (TDEM) data targeting superficial aquifers. High-resolution MT data, acquired between 2011 and 2013, provide information on mid-deep crustal rocks. In addition, new gravity data and joint interpretation of these data sets are undertaken to improve the geological model of the North Perth Basin and test some existing hypotheses. We present a best practice case study and workflow for data acquisition and filtering, robust dimensionality analysis and removal of distortion effects from impedance tensor estimates. 1D and 2D inversions are found to be largely sufficient for the majority of these data while 3D modelling provides an additional tool to verify results. Finally, modelling of gravity data and integrated interpretation ensures robust geological models for the area are consistent with all data available. We conclude with several inferences about the geology in this area. 1) Electromagnetic (EM) and gravity data does not seem to support significant crustal thinning beneath the basin. 2) The Dandaragan Trough appears deeper than generally modelled and 3) extremely high conductivities persist to depth in the basin.

Detection of deep buried metal objects with the UltraTEM

The UltraTEM Deep metal detection system combines a rugged, fast-switching transmitter with a powerful custom designed generator and one to ten three-component receiver coils that collect time-domain electromagnetic induction data across a wide time-range. The roving receiver is operated inside a fixed loop of copper cable, typically 30-150 m long and 5-50 m wide. The large size of the loop results in a slow falloff in primary field with depth and effective excitation of deep buried unexploded ordnance as well as other metal objects such as Ground Engaging Tools (GET). We describe the results of an extensive trial of the UltraTEM system at a magnetically challenging site in Laos. The UltraTEM was able to detect all seeded items buried at depths down to 5 m across a wide range of site conditions. Subsequent work with the system at a different site demonstrated an ability to detect GET to depths of at least 3.0 m in a magnetite stockpile.

Recovering IP information in airborne-time domain electromagnetic data

SUMMARY We propose a methodology to generate a 3D distribution of pseudo-chargeability from airborne time domain electromagnetic data. The processing flow is as follows: (a) Apply 3D inversion to TEM data to restore a background conductivity. This might involve omitting responses that are obviously contaminated with IP signals, such as negative transients in coincident loop surveys. The recovered background conductivity is assumed to be uncontaminated by IP signals. (b) Compute the TEM response from the background conductivity and subtract it from the observations. This yields the d IP data, and reduces the EM coupling. (c) The background conductivity is likely not exactly the earth conductivity, but we assume that the major effects of this inaccuracy will lead to a large scale, smoothly varying perturbation to the d IP data. If this correct, then these can be recognized and removed. (d) The final data are linearly related to a pseudo-chargeability through a sensitivity function that is analogous to that employed in usual DC-IP ground surveys. (e) The d IP data at various time channels can be inverted individually. The pseudo-chargeability models may be useful in themselves or they may be further processed to estimate Cole-Cole, or equivalent, parameters. We demonstrate our procedure on a field data set from Mt. Milligan. In the field example, we identify chargeable targets that show no indication of negative transients in the raw data. From the images we can make inferences about the relative strength and geometries of the chargeable bodies.

Parametric 3D inversion of airborne time domain electromagnetics

Conventional voxel-based inversion algorithms can encounter difficulties when inverting airborne time-domain electromagnetic data in three dimensions. In certain environments with these codes, it can be challenging to delineate sharp boundaries between geologic units with large conductivity contrasts and to accurately image thin conductive targets. Furthermore, spurious circular inversion artifacts, known as ringing, can occur around conductive targets. To address these issues we have developed a parametric inversion code that can be used to find the optimal location, shape and conductivity value of a single anomaly in a homogeneous or heterogeneous background. The algorithm incorporates a Gauss-Newton optimization scheme in conjunction with a level set formulation to outline the anomaly of interest, and can be combined with a conventional voxel-based algorithm in more complicated geologic settings. The code is shown to be successful with a synthetic data set over a thin dipping plate, and two field data sets. For the synthetic scenario, the parametric inversion recovers the true dip and size of a conductive target with no a priori information. The algorithm also accurately defines the extent of a diamondiferous kimberlite pipe and a dipping massive sulphide deposit beneath a conductive overburden.

Stripping very low frequency communication signals with minimum shift keying encoding from streamed time-domain electromagnetic data

The objective of this research is to eliminate very low frequency (VLF) communication signals with frequencies in the range of 15–25 kHz from streamed time-domain electromagnetic data. The frequency-domain approach of notch filtering or bandwidth limitation is unsatisfactory when early delay-time data are required in a time-domain system. United States military VLF transmitters presently communicate [Formula: see text] using minimum shift keying modulation, and it is possible to derive the encrypted bitstream from the data sampled at typical geophysical streaming rates. The method involves convolving the data with waveforms of frequency different by one-quarter of the bit rate above and below the carrier frequency and using the difference between the convolutions to predict the transmitted bits. The transmitted signal is then exactly recreated from the decoded bits, and the predicted signal at the receiver is subtracted from the data stream. It is possible to predict bit rates and encoding methods from other military transmitters through data analysis and again subtract the predicted signals from the streamed data. This procedure reduces the variance of data, implying that unwanted VLF signals have been reduced by a factor of three to nine in stations thousands of kilometers from a VLF transmitter. Much larger signal/noise improvements are predicted for stations within a few hundred kilometers of the VLF source. Lower degrees of improvement are noted from Indian and Chinese transmitters, which appear to have different encoding and modulation methods.