scholarly journals Processing and inversion of commercial helicopter time-domain electromagnetic data for environmental assessments and geologic and hydrologic mapping

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. E149-E159 ◽  
Author(s):  
Joel E. Podgorski ◽  
Esben Auken ◽  
Cyril Schamper ◽  
Anders Vest Christiansen ◽  
Thomas Kalscheuer ◽  
...  
Keyword(s):  
Time Domain ◽  
Mineral Exploration ◽  
Effective Means ◽  
Early Time ◽  
Original Data ◽  
Late Time ◽  
Data Set ◽  
Environmental Assessments ◽  
Time Domain Electromagnetic ◽  
And Inversion

Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semiautomatically identified and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. For example, the average resistivity of the thick surface layer decreased from [Formula: see text] to [Formula: see text], the depths to the layer boundaries were reduced by 15%–23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.

Simple inversion of time‐domain electromagnetic data

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 925-933 ◽  
Author(s):  
C. T. Barnett
Keyword(s):  
Time Domain ◽  
Eddy Currents ◽  
Early Time ◽  
Late Time ◽  
Current Filament ◽  
Equivalent Current ◽  
Time Domain Electromagnetic ◽  
Inversion Procedure ◽  
Current Filaments ◽  
Simple Inversion

The eddy currents induced in a thin confined conductor by a fixed‐loop time‐domain EM system can be represented by a single equivalent current filament. The equivalent current filament stays in the plane of the conductor at all times during the decay of the secondary field, but tends to migrate from a position of maximum primary field coupling at early time toward the center of the conductor at late time. This filament approximation is used in the design of a least‐squares inversion procedure which fits circular or rectangular current filaments to an observed eddy current distribution. The inversion procedure provides a rapid but precise means of estimating the position, size, and attitude of a conductor which has been detected by a time‐domain EM survey.

scholarly journals Transmission compensated primary reflection retrieval in the data domain and consequences for imaging

Geophysics ◽  
2019 ◽  
Vol 84 (4) ◽  
pp. Q27-Q36 ◽  
Author(s):  
Lele Zhang ◽  
Jan Thorbecke ◽  
Kees Wapenaar ◽  
Evert Slob
Keyword(s):  
Thin Layer ◽  
Time Domain ◽  
Velocity Model ◽  
Original Data ◽  
Transmission Losses ◽  
Multiple Reflections ◽  
Data Set ◽  
Numerical Examples ◽  
The One ◽  
The Time Domain

We have developed a scheme that retrieves primary reflections in the two-way traveltime domain by filtering the data. The data have their own filter that removes internal multiple reflections, whereas the amplitudes of the retrieved primary reflections are compensated for two-way transmission losses. Application of the filter does not require any model information. It consists of convolutions and correlations of the data with itself. A truncation in the time domain is applied after each convolution or correlation. The retrieved data set can be used as the input to construct a better velocity model than the one that would be obtained by working directly with the original data and to construct an enhanced subsurface image. Two 2D numerical examples indicate the effectiveness of the method. We have studied bandwidth limitations by analyzing the effects of a thin layer. The presence of refracted and scattered waves is a known limitation of the method, and we studied it as well. Our analysis indicates that a thin layer is treated as a more complicated reflector, and internal multiple reflections related to the thin layer are properly removed. We found that the presence of refracted and scattered waves generates artifacts in the retrieved data.

Three dimensional inversion of multisource time domain electromagnetic data

Geophysics ◽  
2013 ◽  
Vol 78 (1) ◽  
pp. E47-E57 ◽  
Author(s):  
Douglas W. Oldenburg ◽  
Eldad Haber ◽  
Roman Shekhtman
Keyword(s):  
Time Domain ◽  
Maxwell’S Equations ◽  
High Performance ◽  
Maxwell's Equations ◽  
Sulfide Deposit ◽  
3D Inversion ◽  
Data Set ◽  
Electromagnetic Data ◽  
Backward Euler ◽  
Time Domain Electromagnetic

We present a 3D inversion methodology for multisource time-domain electromagnetic data. The forward model consists of Maxwell’s equations in time where the permeability is fixed but electrical conductivity can be highly discontinuous. The goal of the inversion is to recover the conductivity-given measurements of the electric and/or magnetic fields. The availability of matrix-factorization software and high-performance computing has allowed us to solve the 3D time domain EM problem using direct solvers. This is particularly advantageous when data from many transmitters and over many decades are available. We first formulate Maxwell’s equations in terms of the magnetic field, [Formula: see text]. The problem is then discretized using a finite volume technique in space and backward Euler in time. The forward operator is symmetric positive definite and a Cholesky decomposition can be performed with the work distributed over an array of processors. The forward modeling is quickly carried out using the factored operator. Time savings are considerable and they make 3D inversion of large ground or airborne data sets feasible. This is illustrated by using synthetic examples and by inverting a multisource UTEM field data set acquired at San Nicolás, which is a massive sulfide deposit in Mexico.

scholarly journals SN 2012dn from early to late times: 09dc-like supernovae reassessed★

2019 ◽  
Author(s):  
S Taubenberger ◽  
A Floers ◽  
C Vogl ◽  
M Kromer ◽  
J Spyromilio ◽  
...  
Keyword(s):  
Late Phase ◽  
Early Time ◽  
Light Curves ◽  
Optical Data ◽  
Late Time ◽  
Additional Insight ◽  
Data Set ◽  
Type Ia ◽  
Phase Data ◽  
New Criteria

Abstract As a candidate ‘super-Chandrasekhar’ or 09dc-like Type Ia supernova (SN Ia), SN 2012dn shares many characteristics with other members of this remarkable class of objects but lacks their extraordinary luminosity. Here, we present and discuss the most comprehensive optical data set of this SN to date, comprised of a densely sampled series of early-time spectra obtained within the Nearby Supernova Factory project, plus photometry and spectroscopy obtained at the VLT about 1 yr after the explosion. The light curves, colour curves, spectral time series and ejecta velocities of SN 2012dn are compared with those of other 09dc-like and normal SNe Ia, the overall variety within the class of 09dc-like SNe Ia is discussed, and new criteria for 09dc-likeness are proposed. Particular attention is directed to additional insight that the late-phase data provide. The nebular spectra show forbidden lines of oxygen and calcium, elements that are usually not seen in late-time spectra of SNe Ia, while the ionisation state of the emitting iron plasma is low, pointing to low ejecta temperatures and high densities. The optical light curves are characterised by an enhanced fading starting ∼60 d after maximum and very low luminosities in the nebular phase, which is most readily explained by unusually early formation of clumpy dust in the ejecta. Taken together, these effects suggest a strongly perturbed ejecta density profile, which might lend support to the idea that 09dc-like characteristics arise from a brief episode of interaction with a hydrogen-deficient envelope during the first hours or days after the explosion.

3D electromagnetic modeling of graphitic faults in the Athabasca Basin using a finite-volume time-domain approach with unstructured grids

Geophysics ◽  
2021 ◽  
pp. 1-73
Author(s):  
Xushan Lu ◽  
Colin Farquharson ◽  
Jean-Marc Miehé ◽  
Grant Harrison
Keyword(s):  
Finite Volume ◽  
Time Domain ◽  
Tetrahedral Mesh ◽  
Late Time ◽  
Data Set ◽  
Athabasca Basin ◽  
Electric Model ◽  
Modeling Process ◽  
Transient Electromagnetic ◽  
Drilling Data

Uranium exploration in the Athabasca Basin, Canada, relies heavily on ground-based transient electromagnetic (TEM) surveys to target thin, steeply dipping graphitic conductors that are often closely related to the uranium ore deposits. The interpretation of TEM data is important in identifying the locations and trends of conductors in order to guide subsequent drilling campaigns. We present a trial-and-error modeling approach and its application to the interpretation of a data set acquired at Close Lake in the Athabasca Basin. The modeling process has two key tasks: building geo-electric models and computing their TEM responses. The modeling process is repeated with the geo-electric model being iteratively refined based on the match between three-component calculated and measured data from early to late times. To create geo-electric models, we first build a realistic geological model and discretize it using an unstructured tetrahedral mesh, with each mesh cell populated with appropriate resistivities. To calculate the TEM responses of the geo-electric model, we use a 3D finite-volume time-domain (FVTD) algorithm. We construct our initial model based on existing geologic information and drilling data. We show that this modeling process is flexible and can easily handle thin, steeply dipping conductive graphitic fault models with variable resistivities in the fault and background, and with topography. Our interpretation of the Close Lake data matches well with the trend and location of the main conductor as revealed by drilling data, and also confirms the existence of a smaller conductor which only caused noticeable anomalous responses in early-time horizontal-component data. The smaller conductor was suggested by previous electromagnetic data but was missed in a recent interpretation based on the modeling of only late-time vertical component data with plate-based approximate modeling methods.

Electromagnetic mapping of saline contamination at an active brine pit

1996 ◽  
Vol 33 (2) ◽  
pp. 309-323 ◽  
Author(s):  
I J Ferguson ◽  
W J Taylor ◽  
K Schmigel
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Power Lines ◽  
Late Time ◽  
Signal Distortion ◽  
Electromagnetic Methods ◽  
Time Domain Electromagnetic ◽  
Survey Results ◽  
Limited Degree ◽  
Cultural Noise

Frequency-domain and time-domain electromagnetic methods were used to investigate groundwater contamination at an active brine pit in southwestern Manitoba, Canada. The objectives of the survey were to delineate contamination suspected to be occuring at the site and to compare frequency-domain electromagnetic (FDEM) and time-domain electromagnetic (TDEM) measurements in a survey area containing pipelines, fences, and power lines. The survey successfully delineated a region of high conductivity around brine pit, confirming that leakage is occurring from the pit. Modelling of the FDEM results suggests the contamination is spreading within a series of shallow sand units. Comparison of FDEM and TDEM survey results indicate that small-separation FDEM systems are much more useful for mapping in a developed area containing sources of cultural noise. The FDEM systems permit rapid mapping of spatial variations in conductivity, are affected to only a limited degree by cultural features, and provide some resolution of the depth variation of conductivity at shallow depth. It was not possible to obtain useful TDEM measurements anywhere near the active brine pit because of the signal distortion in the late-time response. Key words: geophysics, electromagnetic, brine pit, saline contamination.

scholarly journals tTEM20AAR: a benchmark geophysical data set for unconsolidated fluvioglacial sediments

2021 ◽  
Vol 13 (6) ◽  
pp. 2743-2752
Author(s):  
Alexis Neven ◽  
Pradip Kumar Maurya ◽  
Anders Vest Christiansen ◽  
Philippe Renard
Keyword(s):  
Time Domain ◽  
Reliable Method ◽  
Geophysical Inversion ◽  
Quaternary Deposits ◽  
Data Set ◽  
Time Domain Electromagnetic ◽  
Inversion Techniques ◽  
Alpine Environments ◽  
The Magnetic Field ◽  
Depth Of Investigation

Abstract. Quaternary deposits are complex and heterogeneous. They contain some of the most abundant and extensively used aquifers. In order to improve the knowledge of the spatial heterogeneity of such deposits, we acquired a large (1500 ha) and dense (20 m spacing) time domain electromagnetic (TDEM) data set in the upper Aare Valley, Switzerland (available at https://doi.org/10.5281/zenodo.4269887; Neven et al., 2020). TDEM is a fast and reliable method to measure the magnetic field directly related to the resistivity of the underground. In this paper, we present the inverted resistivity models derived from this acquisition. The depth of investigation ranges between 40 and 120 m, with an average data residual contained in the standard deviation of the data. These data can be used for many different purposes: from sedimentological interpretation of quaternary environments in alpine environments, geological and hydrogeological modeling, to benchmarking geophysical inversion techniques.

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

Geophysics ◽  
2017 ◽  
Vol 82 (2) ◽  
pp. B49-B61 ◽  
Author(s):  
Vladislav Kaminski ◽  
Andrea Viezzoli
Keyword(s):  
Time Domain ◽  
Signal To Noise Ratio ◽  
Mineral Exploration ◽  
Induced Polarization ◽  
Electromagnetic Transients ◽  
Additional Information ◽  
Time Domain Electromagnetic ◽  
Airborne Electromagnetic ◽  
Cole Model ◽  
Inversion Techniques

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.

Inversion of airborne geophysics over the DO-27/DO-18 kimberlites — Part 3: Induced polarization

2017 ◽  
Vol 5 (3) ◽  
pp. T327-T340 ◽  
Author(s):  
Seogi Kang ◽  
Dominique Fournier ◽  
Douglas W. Oldenburg
Keyword(s):  
Time Domain ◽  
Geophysical Methods ◽  
Induced Polarization ◽  
Three Dimensions ◽  
Data Set ◽  
Time Constants ◽  
Airborne Geophysics ◽  
Petrophysical Model ◽  
Time Domain Electromagnetic ◽  
Conductivity Models

The geologically distinct DO-27 and DO-18 kimberlites, often called the Tli Kwi Cho (TKC) kimberlites, have been used as a testbed for airborne geophysical methods applied to kimberlite exploration. This paper focuses on extracting chargeability information from time-domain electromagnetic (TEM) data. Three different TEM surveys, having similar coincident-loop geometry, have been carried out over TKC. Each records negative transients over the main kimberlite units and this is a signature of induced polarization (IP) effects. By applying a TEM-IP inversion workflow to a versatile time domain EM (VTEM) data set we decouple the EM and IP responses in the observations and then recover 3D pseudo-chargeability models at multiple times. A subsequent analysis is used to recover Cole-Cole parameters. Our models demonstrate that both DO-18 and DO-27 pipes are chargeable, but they have different Cole-Cole time constants: 110 and 1160 μs, respectively. At DO-27, we also distinguish between two adjacent kimberlite units based on their respective Cole-Cole time constants. Our chargeability models are combined with the density, magnetic susceptibility and conductivity models to build a 3D petrophysical model of TKC using only information obtained from airborne geophysics. Comparison of this final petrophysical model to a 3D geological model derived from the extensive drilling program demonstrates that we can characterize the three main kimberlite units at TKC: HK, VK, and PK in three dimensions by using airborne geophysics.

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