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

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 flightdependent 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

Incorporating a-priori information into AEM inversion for geological and hydrogeological mapping of the Spiritwood Valley Aquifer, Manitoba, Canada

Buried valleys are important hydrogeological structures in Canada and other glaciated terrains, providing sources of groundwater for drinking, agriculture and industrial applications. Hydrgeological exploration methods such as pumping tests, boreholes coring or ground-based geophysical methods (seismic and electrical resistivity tomography) provide limited spatial information and are inadequate to efficiently predict the sustainability of these aquifers at the regional scale. Airborne geophysics can be used to significantly improve geological and hydrogeological knowledge on a regional scale. There has been demonstrated success at using airborne electromagnetics for mapping and characterization of buried valleys in different geological contexts (Auken et al., 2008; Jørgensen et al., 2003; Jørgensen et al., 2009; Steuer et al., 2009). Despite the fact that both electromagnetic surveys and reflection seismic profiling are used extensively in hydrogeological mapping, integration of the methods is a relatively unexplored discipline (Høyer et al., 2011). The Spiritwood Valley is a Canada-USA trans-border buried valley aquifer that runs approximately NW – SE and extends 500 km from Manitoba, across North Dakota and into South Dakota (Winter et al., 1984). The Spiritwood aquifer system consists of glacially deposited silt and clay with sand and gravel bodies, infilling a broad north-south trending valley that has been identified primarily based on water wells information (Wiecek, 2009). The valley is incised into bedrock consisting of fractured siliceous shale. As part of its Groundwater Geoscience Program, the Geological Survey of Canada (GSC) has been investigating buried valley aquifers in Canada using airborne and ground-based geophysical techniques. To obtain a regional three-dimensional assessment of complex aquifer geometries for the Spiritwood, both geophysical and geological investigations were performed with the aim to develop an integrated conceptual model for a quantitative description of the aquifer system. In 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AeroTEM III) survey over a 1062 km2 area along the Spiritwood Valley, north of the US border (Oldenborger 2010a, 2010b). AEM inversion results show multiple resistive valley features inside a wider, more conductive valley structure within the conductive bedrock (Fig. 1). Furthermore, the complexity of the geometries, spatial distribution and size of the channels is evident. Other ground based data collected in the survey area make it possible to provide some constraints on the AEM resistivity model. Downhole resistivity logs were collected that provide information on the electrical model relative to the geological layers (Crow et al., 2012). In addition, over 10 line-km of electrical resistivity data and 42 km of high resolution landstreamer seismic reflection data (Figs. 2a, 2b) were collected at selected sites (Oldenborger et al., 2012). In this short paper we present results obtained from the data inversion and an example of integration of ancillary seismic data into the AEM inversion. In particular, the elevation to a layer (shale bedrock elevation) as interpreted from seismic is added to the inversion to constrain the resistivity model

3D Hydrogeological Model Building Using Airborne Electromagnetic Data

We develop a 3D geological modelling procedure supported by the combination of helicopter time-domain electromagnetic data, seismic reflection data, and water well records for the Spiritwood buried valley aquifer system in Manitoba, Canada. Our procedure is an innovative hybrid of knowledge-driven and data-driven schemes that provides a clear protocol for incorporating different types of geophysical data into a 3D stratigraphic model framework. The limited spatial density of water well bedrock observations precludes detection of the buried valley bedrock topography and renders the water well records alone inadequate for accurate hydrogeological model building. The expert interpretation of the geophysical data allows for leveraging of a spatially extensive dataset with rich information content that would be otherwise difficult to utilize for lithostratigraphic classification

Recent AEM Case Study Examples of a Full Waveform Time-Domain System for Near-Surface and Groundwater Applications

Early time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas but this poses difficulties due to a variety of issues, such as system bandwidth, system calibration and parasitic loop capacitance. In an effort to address this issue, a continued system design strategy, aimed at improving its early-channel VTEM data, has achieved fully calibrated, quantitative measurements closer to the transmitter current turn-off, while maintaining reasonably optimal deep penetration characteristics. The new design implementation, known as “Full Waveform” VTEM was previously described by Legault et al. (2012). This paper presents some case-study examples of a Full Waveform helicopter time-domain EM system for near-surface application

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

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 flightdependent 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.Published61-727A. Geofisica di esplorazioneJCR Journalpartially_ope

Incorporating a-priori information into AEM inversion for geological and hydrogeological mapping of the Spiritwood Valley Aquifer, Manitoba, Canada

Buried valleys are important hydrogeological structures in Canada and other glaciated terrains, providing sources of groundwater for drinking, agriculture and industrial applications. Hydrgeological exploration methods such as pumping tests, boreholes coring or ground-based geophysical methods (seismic and electrical resistivity tomography) provide limited spatial information and are inadequate to efficiently predict the sustainability of these aquifers at the regional scale. Airborne geophysics can be used to significantly improve geological and hydrogeological knowledge on a regional scale. There has been demonstrated success at using airborne electromagnetics for mapping and characterization of buried valleys in different geological contexts (Auken et al., 2008; Jørgensen et al., 2003; Jørgensen et al., 2009; Steuer et al., 2009). Despite the fact that both electromagnetic surveys and reflection seismic profiling are used extensively in hydrogeological mapping, integration of the methods is a relatively unexplored discipline (Høyer et al., 2011). The Spiritwood Valley is a Canada-USA trans-border buried valley aquifer that runs approximately NW – SE and extends 500 km from Manitoba, across North Dakota and into South Dakota (Winter et al., 1984). The Spiritwood aquifer system consists of glacially deposited silt and clay with sand and gravel bodies, infilling a broad north-south trending valley that has been identified primarily based on water wells information (Wiecek, 2009). The valley is incised into bedrock consisting of fractured siliceous shale. As part of its Groundwater Geoscience Program, the Geological Survey of Canada (GSC) has been investigating buried valley aquifers in Canada using airborne and ground-based geophysical techniques. To obtain a regional three-dimensional assessment of complex aquifer geometries for the Spiritwood, both geophysical and geological investigations were performed with the aim to develop an integrated conceptual model for a quantitative description of the aquifer system. In 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AeroTEM III) survey over a 1062 km2 area along the Spiritwood Valley, north of the US border (Oldenborger 2010a, 2010b). AEM inversion results show multiple resistive valley features inside a wider, more conductive valley structure within the conductive bedrock (Fig. 1). Furthermore, the complexity of the geometries, spatial distribution and size of the channels is evident. Other ground based data collected in the survey area make it possible to provide some constraints on the AEM resistivity model. Downhole resistivity logs were collected that provide information on the electrical model relative to the geological layers (Crow et al., 2012). In addition, over 10 line-km of electrical resistivity data and 42 km of high resolution landstreamer seismic reflection data (Figs. 2a, 2b) were collected at selected sites (Oldenborger et al., 2012). In this short paper we present results obtained from the data inversion and an example of integration of ancillary seismic data into the AEM inversion. In particular, the elevation to a layer (shale bedrock elevation) as interpreted from seismic is added to the inversion to constrain the resistivity model.Published194 - 1997A. Geofisica di esplorazioneN/A or not JCRope

Incorporating ancillary data into the inversion of airborne time-domain electromagnetic data for hydrogeological applications

Helicopter time-domain electromagnetic (HTEM) surveys often suffer fromsignificant inaccuracies in the early-time or near-surface data—a problem that can lead to errors in the inversemodel or limited near-surface resolution in the event that early time gates are removed. We present an example illustrating the use of seismic data to constrain the model recovered from an HTEM survey over the Spiritwood buried valley aquifer in Manitoba, Canada. The incorporation of seismic reflection surfaces results in improved near-surface resistivity in addition to a more continuous bedrock interface with a sharper contact. The seismic constraints reduce uncertainty in the resistivity values of the overlying layers, although no a priori information is added directly to those layers. Subsequently, we use electrical resistivity tomography (ERT) and borehole data to verify the constrained HTEM models. Treating the ERT and borehole logs as reference information, we perform an iterative time-shift calibration of the HTEM soundings to achieve regional-scale consistency between the recovered HTEM models and the reference information. Given the relatively small time-shifts employed, this calibration procedure most significantly affects the early-time data and brings the first useable time gate to a time earlier than the nominal first gate after ramp off. Although time shifts are small, changes in the model are observed from the near-surface to depths of 100 m. Calibration is combined with seismic constraints to achieve amodel with the greatest level of consistency between data sets and, thus, the greatest degree of confidence. For the Spiritwood buried valley, calibrated and constrained models reveal more structure in the valley-fill sediments and increased continuity of the bedrock contact.Published35 - 437A. Geofisica di esplorazioneJCR Journalrestricte

Incorporating ancillary data into the inversion of airborne time-domain electromagnetic data for hydrogeological applications

Helicopter time-domain electromagnetic (HTEM) surveys often suffer fromsignificant inaccuracies in the early-time or near-surface data—a problem that can lead to errors in the inversemodel or limited near-surface resolution in the event that early time gates are removed. We present an example illustrating the use of seismic data to constrain the model recovered from an HTEM survey over the Spiritwood buried valley aquifer in Manitoba, Canada. The incorporation of seismic reflection surfaces results in improved near-surface resistivity in addition to a more continuous bedrock interface with a sharper contact. The seismic constraints reduce uncertainty in the resistivity values of the overlying layers, although no a priori information is added directly to those layers. Subsequently, we use electrical resistivity tomography (ERT) and borehole data to verify the constrained HTEM models. Treating the ERT and borehole logs as reference information, we perform an iterative time-shift calibration of the HTEM soundings to achieve regional-scale consistency between the recovered HTEM models and the reference information. Given the relatively small time-shifts employed, this calibration procedure most significantly affects the early-time data and brings the first useable time gate to a time earlier than the nominal first gate after ramp off. Although time shifts are small, changes in the model are observed from the near-surface to depths of 100 m. Calibration is combined with seismic constraints to achieve amodel with the greatest level of consistency between data sets and, thus, the greatest degree of confidence. For the Spiritwood buried valley, calibrated and constrained models reveal more structure in the valley-fill sediments and increased continuity of the bedrock contact

The Impact on Geological and Hydrogeological Mapping Results of Moving from Ground to Airborne TEM

In the past three decades, airborne electromagnetic (AEM) systems have been used for many groundwater exploration purposes. This contribution of airborne geophysics for both groundwater resource mapping and water quality evaluations and management has increased dramatically over the past ten years, proving how these systems are appropriate for large-scale and efficient groundwater surveying. One of the major reasons for its popularity is the time and cost efficiency in producing spatially extensive datasets that can be applied to multiple purposes. In this paper, we carry out a simple, yet rigorous, simulation showing the impact of an AEM dataset towards hydrogeological mapping, comparing it to having only a ground-based transient electromagnetic (TEM) dataset (even if large and dense), and to having only boreholes. We start from an AEM survey and then simulate two different ground TEM datasets: a high resolution survey and a reconnaissance survey. The electrical resistivity model, which is the final geophysical product after data processing and inversion, changes with different levels of data density. We then extend the study to describe the impact on the geological and hydrogeological output models, which can be derived from these different geophysical results, and the potential consequences for groundwater management. Different data density results in significant differences not only in the spatial resolution of the output resistivity model, but also in the model uncertainty, the accuracy of geological interpretations and, in turn, the appropriateness of groundwater management decisions. The AEM dataset provides high resolution results and well-connected geological interpretations, which result in a more detailed and confident description of all of the existing geological structures. In contrast, a low density dataset from a ground-based TEM survey yields low resolution resistivity models, and an uncertain description of the geological setting

The Impact on Geological and Hydrogeological Mapping Results of Moving from Ground to Airborne TEM

In the past three decades, airborne electromagnetic (AEM) systems have been used for many groundwater exploration purposes. This contribution of airborne geophysics for both groundwater resource mapping and water quality evaluations and management has increased dramatically over the past ten years, proving how these systems are appropriate for large-scale and efficient groundwater surveying. One of the major reasons for its popularity is the time and cost efficiency in producing spatially extensive datasets that can be applied to multiple purposes. In this paper, we carry out a simple, yet rigorous, simulation showing the impact of an AEM dataset towards hydrogeological mapping, comparing it to having only a ground-based transient electromagnetic (TEM) dataset (even if large and dense), and to having only boreholes. We start from an AEM survey and then simulate two different ground TEM datasets: a high resolution survey and a reconnaissance survey. The electrical resistivity model, which is the final geophysical product after data processing and inversion, changes with different levels of data density. We then extend the study to describe the impact on the geological and hydrogeological output models, which can be derived from these different geophysical results, and the potential consequences for groundwater management. Different data density results in significant differences not only in the spatial resolution of the output resistivity model, but also in the model uncertainty, the accuracy of geological interpretations and, in turn, the appropriateness of groundwater management decisions. The AEM dataset provides high resolution results and well-connected geological interpretations, which result in a more detailed and confident description of all of the existing geological structures. In contrast, a low density dataset from a ground-based TEM survey yields low resolution resistivity models, and an uncertain description of the geological setting.Published53-667A. Geofisica di esplorazioneJCR Journalrestricte