Results of Recent VTEM Helicopter System Development Testing over the Spiritwood Valley Aquifer, Manitoba

Legault, Jean M.; Prikhodko, Alexander; Dodds, D. J.; Macnae, James; Oldenborger, Greg A.

doi:10.4133/1.4721712

Cited by 21 publications

(16 citation statements)

References 14 publications

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“…, Legault et al . ). The AeroTEM calibration procedure is adapted from that applied in Denmark to AEM data whereby an electrical reference model is derived from ground TEM data (Foged et al .…”

Section: Methodsmentioning

confidence: 97%

“…For calibration purposes, we employ a "full waveform" VTEM dataset collected by Geotech Ltd. over a small portion of the AeroTEM survey area ( Fig. 1, Legault et al 2012). The AeroTEM calibration procedure is adapted from that applied in Denmark to AEM data whereby an electrical reference model is derived from ground TEM data (Foged et al 2013).…”

Section: Introductionmentioning

confidence: 99%

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Joining multiple AEM datasets to improve accuracy, cross calibration and derived products: The Spiritwood VTEM and AeroTEM case study

Sapia

Viezzoli

Oldenborger

2010

Near Surface Geophysics

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Airborne time‐domain electromagnetic methods (AEM) are useful for hydrogeological mapping due to their rapid and extensive spatial coverage and high correlation between measured magnetic fields, electrical conductivity, and relevant hydrogeological parameters. However, AEM data, preprocessing and modelling procedures can suffer from inaccuracies that may dramatically affect the final interpretation. We demonstrate the importance and the benefits of advanced data processing for two AEM datasets (AeroTEM III and VTEM) collected over the Spiritwood buried valley aquifer in southern Manitoba, Canada. Early‐time data gates are identified as having significant flight‐dependent signal bias that reflects survey flights and flight lines. These data are removed from inversions along with late time data gates contaminated by apparently random noise. In conjunction with supporting information, the less‐extensive, but broader‐band VTEM data are used to construct an electrical reference model. The reference model is subsequently used to calibrate the AeroTEM dataset via forward modelling for coincident soundings. The procedure produces calibration factors that we apply to AeroTEM data over the entire survey domain. Inversion of the calibrated data results in improved data fits, particularly at early times, but some flight‐line artefacts remain. Residual striping between adjacent flights is corrected by including a mean empirical amplitude correction factor within the spatially constrained inversion scheme. Finally, the AeroTEM and VTEM data are combined in a joint inversion. Results confirm consistency between the two different AEM datasets and the recovered models. On the contrary, joint inversion of unprocessed or uncalibrated AEM datasets results in erroneous resistivity models which, in turn, can result in an inappropriate hydrogeological interpretation of the study area.

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Section: Methodsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Joining multiple AEM datasets to improve accuracy, cross calibration and derived products: The Spiritwood VTEM and AeroTEM case study

Sapia

Viezzoli

Oldenborger

2010

Near Surface Geophysics

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“…Another CR handling technique applied to the helicopter‐borne AEM system VTEM was presented in an abstract by Macnae and Baron‐Hay () and applied to a survey with a newly designed VTEM system by Legault et al . (). It has to be noted that the two approaches are fundamentally different and have been developed independently.…”

Section: Introductionmentioning

confidence: 97%

“…The method presented for handling CR was first reported in an abstract by Auken, Foged and Sørensen (2010) and after obtaining a few years' experience with field data it is now the subject of this paper. Another CR handling technique applied to the helicopter-borne AEM system VTEM was presented in an abstract by Macnae and Baron-Hay (2010) and applied to a survey with a newly designed VTEM system by Legault et al (2012). It has to be noted that the two approaches are fundamentally different and have been developed independently.…”

Section: Introductionmentioning

confidence: 99%

Coil response inversion for very early time modelling of helicopter‐borne time‐domain electromagnetic data and mapping of near‐surface geological layers

Schamper

Auken

Sørensen

2014

Geophysical Prospecting

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International audienceVery early times in the order of 2-3 s from the end of the turn-off ramp for time-domain electromagnetic systems are crucial for obtaining a detailed resolution of the near-surface geology in the depth interval 0-20 m. For transient electromagnetic systems working in the off time, an electric current is abruptly turned off in a large transmitter loop causing a secondary electromagnetic field to be generated by the eddy currents induced in the ground. Often, however, there is still a residual primary field generated by remaining slowly decaying currents in the transmitter loop. The decay disturbs or biases the earth response data at the very early times. These biased data must be culled, or some specific processing must be applied in order to compensate or remove the residual primary field. As the bias response can be attributed to decaying currents with its time constantly controlled by the geometry of the transmitter loop, we denote it the Coil Response'. The modelling of a helicopter-borne time-domain system by an equivalent electronic circuit shows that the time decay of the coil response remains identical whatever the position of the receiver loop, which is confirmed by field measurements. The modelling also shows that the coil response has a theoretical zero location and positioning the receiver coil at the zero location eliminates the coil response completely. However, spatial variations of the coil response around the zero location are not insignificant and even a few cm deformation of the carrier frame will introduce a small coil response. Here we present an approach for subtracting the coil response from the data by measuring it at high altitudes and then including an extra shift factor into the inversion scheme. The scheme is successfully applied to data from the SkyTEM system and enables the use of very early time gates, as early as 2-3 s from the end of the ramp, or 5-6 s from the beginning of the ramp. Applied to a large-scale airborne electromagnetic survey, the coil response compensation provides airborne electromagnetic methods with a hitherto unseen good resolution of shallow geological layers in the depth interval 0-20 m. This is proved by comparing results from the airborne electromagnetic survey to more than 100 km of Electrical Resistivity Tomography measured with 5 m electrode spacing

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“…The first approach, based on the observation that, in some cases the ideal earth response can be modeled by a finite sum of exponential decays of varying amplitude and time constant, restricts the recovered earth response to reside in a subspace of such functions (Wolfgram and Karlik, 1995;Stolz and Macnae, 1998). The second approach uses the fact that periodic convolution in the discrete time domain corresponds to multiplication in the discrete frequency domain (Macnae and Lamontagne, 1982;Annan, 1986;Lane et al, 2000;Sattel et al, 2004;Macnae and Baron-Hay, 2010;Legault et al, 2012;Sattel and Battig, 2016). Thus, because the time-domain system response is normally periodic in practice, deconvolution corresponds to division in the frequency domain.…”

Section: Introductionmentioning

confidence: 99%

Noise properties of Fourier deconvolution for time-domain electromagnetic soundings

Rasmussen

Nyboe²,

Mai³

et al. 2017

GEOPHYSICS

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The transient electromagnetic method (TEM) is widely used for mapping the earth's subsurface resistivity structures, e.g., for groundwater or mineral exploration, using airborne and ground-based systems. Data from TEM surveys can be modified using Fourier deconvolution to correct for instrument drift, increase apparent bandwidth, or alter the apparent excitation waveform, e.g., to fuse or compare measurements from different systems, for visual presentation purposes, or to make the data compatible with a specific processing or inversion code. Although this method has been applied in several studies, little attention has been devoted to its properties with regard to noise. Using a generic analytical system model and examples featuring synthetic and field data, we perform a detailed analysis of the noise properties of Fourier deconvolution in the context of TEM. We find that although the effects from stationary noise are trivial, effects from nonstationary noise are less intuitive and more severe, e.g., causing on-time noise phenomena to degrade off-time data. In general, we observe that the method decreases the signal-to-noise ratio, and our recommendation is therefore that Fourier deconvolution should only be applied when it is desirable for presentation purposes or when strictly necessary for processing or inversion.

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Results of Recent VTEM Helicopter System Development Testing over the Spiritwood Valley Aquifer, Manitoba

Cited by 21 publications

References 14 publications

Joining multiple AEM datasets to improve accuracy, cross calibration and derived products: The Spiritwood VTEM and AeroTEM case study

Joining multiple AEM datasets to improve accuracy, cross calibration and derived products: The Spiritwood VTEM and AeroTEM case study

Coil response inversion for very early time modelling of helicopter‐borne time‐domain electromagnetic data and mapping of near‐surface geological layers

Noise properties of Fourier deconvolution for time-domain electromagnetic soundings

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